powershell: fix prepend for new variables and fix unsetenv for non-ex…

…isting variables (#1477)

* Fix env prepend failing for new env vars @SitiSchu
* implement modify existing and add new test to exercise rex usage of 'unset' and 'prependenv' @maxnbk
* extend same prepend/append logic to powershell unset @maxnbk

---------

Signed-off-by: SitiSchu <admin@sitischu.com>
Signed-off-by: Stephen Mackenzie <maxnbk@users.noreply.github.com>
Co-authored-by: SitiSchu <admin@sitischu.com>

src/rez/tests/test_shells.py

@@ -419,6 +419,7 @@ def _rex_appending():
             from rez.shells import create_shell
             sh = create_shell()
 
+            env.FOO.unset()
             env.FOO.append("hey")
             info(sh.get_key_token("FOO"))
             env.FOO.append(literal("$DAVE"))
@@ -434,6 +435,26 @@ def _rex_appending():
 
         _execute_code(_rex_appending, expected_output)
 
+        def _rex_prepending():
+            from rez.shells import create_shell
+            sh = create_shell()
+
+            env.FOO.unset()
+            env.FOO.prepend("hey")
+            info(sh.get_key_token("FOO"))
+            env.FOO.prepend(literal("$DAVE"))
+            info(sh.get_key_token("FOO"))
+            env.FOO.prepend("Dave's not here man")
+            info(sh.get_key_token("FOO"))
+
+        expected_output = [
+            "hey",
+            sh.pathsep.join(["$DAVE", "hey"]),
+            sh.pathsep.join(["Dave's not here man", "$DAVE", "hey"])
+        ]
+
+        _execute_code(_rex_prepending, expected_output)
+
     @per_available_shell()
     def test_rex_code_alias(self, shell):
         """Ensure PATH changes do not influence the alias command.

src/rezplugins/shell/_utils/powershell_base.py

@@ -264,8 +264,8 @@ def prependenv(self, key, value):
         # Be careful about ambiguous case in pwsh on Linux where pathsep is :
         # so that the ${ENV:VAR} form has to be used to not collide.
         self._addline(
-            'Set-Item -Path "Env:{0}" -Value ("{1}{2}" + (Get-ChildItem "Env:{0}").Value)'.format(
-                key, value, self.pathsep)
+            'Set-Item -Path "Env:{0}" -Value ("{1}{2}" + (Get-ChildItem -ErrorAction SilentlyContinue "Env:{0}").Value)'
+            .format(key, value, self.pathsep)
         )
 
     def appendenv(self, key, value):
@@ -280,7 +280,9 @@ def appendenv(self, key, value):
             .format(key, os.path.pathsep, value))
 
     def unsetenv(self, key):
-        self._addline(r"Remove-Item Env:\%s" % key)
+        self._addline(
+            'Remove-Item -ErrorAction SilentlyContinue "Env:{0}"'.format(key)
+        )
 
     def resetenv(self, key, value, friends=None):
         self._addline(self.setenv(key, value))