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
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<div class='path'>path: <a href='/guix/log/?id=0a078b5a31f0c93bbd97f153828da4c14fe3429a'>root</a>/<a href='/guix/log/gnu?id=0a078b5a31f0c93bbd97f153828da4c14fe3429a'>gnu</a>/<a href='/guix/log/gnu/installer?id=0a078b5a31f0c93bbd97f153828da4c14fe3429a'>installer</a>/<a href='/guix/log/gnu/installer/substitutes.scm?id=0a078b5a31f0c93bbd97f153828da4c14fe3429a'>substitutes.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/substitutes.scm?id=0a078b5a31f0c93bbd97f153828da4c14fe3429a&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-12-11 21:37:07 +0100'>2020-12-11</span></td><td><a href='/guix/commit/gnu/installer/substitutes.scm?id=8361817bf693742757b096468198626f297bb09e'>install: Discover local substitute servers.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/installer/substitutes.scm: New file.
* gnu/installer/newt/substitutes.scm: New file.
* gnu/local.mk (INSTALLER_MODULES): Add them.
* po/guix/POTFILES.in: Add gnu/installer/newt/substitutes.scm.
* gnu/installer/proxy.scm (with-silent-shepherd): Move to ...
* gnu/installer/utils.scm: ... here.
* gnu/installer/record.scm (&lt;installer&gt;)[substitutes-page]: New field.
* gnu/installer/newt.scm (substitutes-page): New procedure,
(newt-installer): register it.
* gnu/installer.scm (installer-steps): Add "substitutes-page" step.
* gnu/system/install.scm (%installation-services): Add avahi-service-type and
enable substitute server discover in guix-service-type.
[&lt;name-service-switch&gt;]: Set it to %mdns-host-lookup-nss.
</span></span></td><td>Mathieu Othacehe</td></tr>