Look for extensions in $CHROMIUM_EXTENSION_DIRECTORY instead of
/usr/share/chromium/extensions.

--- a/chrome/common/chrome_paths.cc
+++ b/chrome/common/chrome_paths.cc
@@ -4,6 +4,7 @@
 
 #include "chrome/common/chrome_paths.h"
 
+#include "base/environment.h"
 #include "base/files/file_util.h"
 #include "base/logging.h"
 #include "base/native_library.h"
@@ -511,7 +512,13 @@
 #endif
 #if defined(OS_LINUX) || defined(OS_CHROMEOS)
     case chrome::DIR_STANDALONE_EXTERNAL_EXTENSIONS: {
-      cur = base::FilePath(kFilepathSinglePrefExtensions);
+      std::unique_ptr<base::Environment> environment(base::Environment::Create());
+      std::string extension_dir;
+      if (environment->GetVar("CHROMIUM_EXTENSION_DIRECTORY", &extension_dir)) {
+        cur = base::FilePath(extension_dir);
+      } else {
+        cur = base::FilePath(kFilepathSinglePrefExtensions);
+      }
       break;
     }
 #endif
's customized Guix</td><td class='sub right'></td></tr></table>
<table class='tabs'><tr><td>
<a href='/guix/about/'>about</a><a href='/guix/'>summary</a><a href='/guix/refs/?id=4498894b1b2b959e3c87d947659cbd5cc069df2e'>refs</a><a class='active' href='/guix/log/gnu/installer/newt/keymap.scm'>log</a><a href='/guix/tree/gnu/installer/newt/keymap.scm?id=4498894b1b2b959e3c87d947659cbd5cc069df2e'>tree</a><a href='/guix/commit/gnu/installer/newt/keymap.scm?id=4498894b1b2b959e3c87d947659cbd5cc069df2e'>commit</a><a href='/guix/diff/gnu/installer/newt/keymap.scm?id=4498894b1b2b959e3c87d947659cbd5cc069df2e'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/gnu/installer/newt/keymap.scm'>
<input type='hidden' name='id' value='4498894b1b2b959e3c87d947659cbd5cc069df2e'/><select name='qt'>
<option value='grep'>log msg</option>
<option value='author'>author</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=4498894b1b2b959e3c87d947659cbd5cc069df2e'>root</a>/<a href='/guix/log/gnu?id=4498894b1b2b959e3c87d947659cbd5cc069df2e'>gnu</a>/<a href='/guix/log/gnu/installer?id=4498894b1b2b959e3c87d947659cbd5cc069df2e'>installer</a>/<a href='/guix/log/gnu/installer/newt?id=4498894b1b2b959e3c87d947659cbd5cc069df2e'>newt</a>/<a href='/guix/log/gnu/installer/newt/keymap.scm?id=4498894b1b2b959e3c87d947659cbd5cc069df2e'>keymap.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/gnu/installer/newt/keymap.scm?id=4498894b1b2b959e3c87d947659cbd5cc069df2e&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2021-01-12 16:40:27 +0100'>2021-01-12</span></td><td><a href='/guix/commit/gnu/installer/newt/keymap.scm?id=bb4e67415eb6d9871ee1b50b0a01e08c19f4809a'>installer: Fix crash in parameters menu.</a><span class='msg-avail'>...</span></td><td>Mathieu Othacehe</td></tr>
<tr><td><span title='2020-04-09 03:45:22 +0000'>2020-04-09</span></td><td><a href='/guix/commit/gnu/installer/newt/keymap.scm?id=7bc71025ccd0fabd739803889825e10341fbabdf'>installer: Include empty variant in keyboard layout selection.</a><span class='msg-avail'>...</span></td><td>Florian Pelz</td></tr>
<tr><td><span title='2020-04-09 02:42:54 +0000'>2020-04-09</span></td><td><a href='/guix/commit/gnu/installer/newt/keymap.scm?id=91c231a2223440081426929828a23c7baa0214fd'>installer: Allow Alt+Shift toggle from non-Latin keyboard layouts.</a><span class='msg-avail'>...</span></td><td>Florian Pelz</td></tr>
<tr><td><span title='2020-04-08 10:24:02 +0200'>2020-04-08</span></td><td><a href='/guix/commit/gnu/installer/newt/keymap.scm?id=07a53bd512530d8f87e076263227216c467727bb'>installer: Turn help menu into parameters menu.</a><span class='msg-avail'>...</span></td><td>Mathieu Othacehe</td></tr>
<tr><td><span title='2020-04-06 15:06:03 +0200'>2020-04-06</span></td><td><a href='/guix/commit/gnu/installer/newt/keymap.scm?id=786c9c39bc0826737e99f0c750ea30033cdbc202'>installer: Add a help page.</a><span class='msg-avail'>...</span></td><td>Mathieu Othacehe</td></tr>