Inherit essential environment variables in tests.

Note: it could be better to generalize this in SCons/Platform/posix.py
instead of just patching the tests.

diff --git a/SCons/ActionTests.py b/SCons/ActionTests.py
--- a/SCons/ActionTests.py
+++ b/SCons/ActionTests.py
@@ -98,6 +98,7 @@ outfile2 = test.workpath('outfile2')
 pipe_file = test.workpath('pipe.out')
 
 scons_env = SCons.Environment.Environment()
+scons_env['ENV']['PATH'] += os.environ['PATH']
 
 # Capture all the stuff the Actions will print,
 # so it doesn't clutter the output.
@@ -1090,6 +1091,8 @@ class CommandActionTestCase(unittest.TestCase):
         except AttributeError:
             env = Environment()
 
+        env = Environment(ENV={'PATH': os.environ['PATH']})
+
         cmd1 = r'%s %s %s xyzzy' % (_python_, act_py, outfile)
 
         act = SCons.Action.CommandAction(cmd1)
@@ -1884,7 +1887,7 @@ class ListActionTestCase(unittest.TestCase):
                     f.write("class2b\n")
 
         act = SCons.Action.ListAction([cmd2, function2, class2a(), class2b])
-        r = act([], [], Environment(out=outfile))
+        r = act([], [], Environment(out=outfile, ENV={'PATH' : os.getenv('PATH')}))
         assert isinstance(r.status, class2b), r.status
         c = test.read(outfile, 'r')
         assert c == "act.py: 'syzygy'\nfunction2\nclass2a\nclass2b\n", c
@@ -1948,7 +1951,7 @@ class LazyActionTestCase(unittest.TestCase):
         a([], [], env=Environment(BAR=f, s=self))
         assert self.test == 1, self.test
         cmd = r'%s %s %s lazy' % (_python_, act_py, outfile)
-        a([], [], env=Environment(BAR=cmd, s=self))
+        a([], [], env=Environment(BAR=cmd, s=self, ENV={'PATH' : os.getenv('PATH')}))
         c = test.read(outfile, 'r')
         assert c == "act.py: 'lazy'\n", c
 
diff --git a/SCons/SConfTests.py b/SCons/SConfTests.py
--- a/SCons/SConfTests.py
+++ b/SCons/SConfTests.py
@@ -71,7 +71,9 @@ class SConfTestCase(unittest.TestCase):
         # and we need a new environment, cause references may point to
         # old modules (well, at least this is safe ...)
         self.scons_env = self.Environment.Environment()
-        self.scons_env.AppendENVPath('PATH', os.environ['PATH'])
+        # Inherit the OS environment to get essential variables.
+        inherited_env = os.environ.copy()
+        self.scons_env['ENV'] = inherited_env
 
         # we want to do some autodetection here
         # this stuff works with
ss='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/gnu/services/getmail.scm?id=8c8fb5c878b0efc02dc92e31e90d40fc4d850937&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2022-12-02 00:02:12 +0100'>2022-12-02</span></td><td><a href='/guix/commit/gnu/services/getmail.scm?id=7c1161dba4c8930d3a500a72ccaaaf50df73f466'>records: 'match-record' checks fields at macro-expansion time.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2022-11-17 23:03:50 +0100'>2022-11-17</span></td><td><a href='/guix/commit/gnu/services/getmail.scm?id=83c9e00ffbd41776c003f6992e9d613a5434fff9'>services: getmail: Use 'match-record'.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2022-08-25 00:10:32 -0400'>2022-08-25</span></td><td><a href='/guix/commit/gnu/services/getmail.scm?id=ee08277a70d29b6d707524a4e17620f50b87adf4'>services: Use the new maybe/unset API.</a><span class='msg-avail'>...</span></td><td>Attila Lendvai</td></tr>
<tr><td><span title='2021-11-30 01:08:55 +0100'>2021-11-30</span></td><td><a href='/guix/commit/gnu/services/getmail.scm?id=892f1b7273d57b25940700877a02618fe826cc08'>services: Accept &lt;inferior-package&gt;s in lieu of &lt;package&gt;s.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>