fix(powershell): execution policy check breaks when run in pwsh

When topgrade is run from within pwsh, the execution policy check breaks
for the Windows Update and Windows Store steps, because they use normal
powershell and the inherited PSModulePath environment variable breaks
the Microsoft.PowerShell.Security module import. So we unset that
variable to fix the issue, but also allow for those steps to use pwsh as
neither step actually requires PowerShell 5.

Co-authored-by: nistee <52573120+niStee@users.noreply.github.com>

src/steps/powershell.rs

@@ -34,7 +34,7 @@ impl Powershell {
             .or_else(|| which("powershell").map(|p| (Some(p), false)))
             .unwrap_or((None, false));
 
-        let profile = path.as_ref().and_then(Self::get_profile);
+        let profile = path.as_ref().and_then(|path| Self::get_profile(path, is_pwsh));
 
         Self { path, profile, is_pwsh }
     }
@@ -43,29 +43,12 @@ impl Powershell {
         self.path.is_some()
     }
 
-    #[cfg(windows)]
-    pub fn windows_powershell() -> Self {
-        if terminal::is_dumb() {
-            return Self {
-                path: None,
-                profile: None,
-                is_pwsh: false,
-            };
-        }
-
-        Self {
-            path: which("powershell"),
-            profile: None,
-            is_pwsh: false,
-        }
-    }
-
     pub fn profile(&self) -> Option<&PathBuf> {
         self.profile.as_ref()
     }
 
-    fn get_profile(path: &PathBuf) -> Option<PathBuf> {
-        let profile = Self::build_command_internal(path, "Split-Path $PROFILE")
+    fn get_profile(path: &PathBuf, is_pwsh: bool) -> Option<PathBuf> {
+        let profile = Self::build_command_internal(path, is_pwsh, "Split-Path $PROFILE")
             .output_checked_utf8()
             .map(|output| output.stdout.trim().to_string())
             .and_then(|s| PathBuf::from(s).require())
@@ -75,12 +58,18 @@ impl Powershell {
     }
 
     /// Builds an "internal" powershell command
-    fn build_command_internal(path: &PathBuf, cmd: &str) -> Command {
+    fn build_command_internal(path: &PathBuf, is_pwsh: bool, cmd: &str) -> Command {
         let mut command = Command::new(path);
 
         command.args(["-NoProfile", "-Command"]);
         command.arg(cmd);
 
+        // If topgrade was run from pwsh, but we are trying to run powershell, then
+        // the inherited PSModulePath breaks module imports
+        if !is_pwsh {
+            command.env_remove("PSModulePath");
+        }
+
         command
     }
 
@@ -106,6 +95,12 @@ impl Powershell {
         command.args(["-NoProfile", "-Command"]);
         command.arg(cmd);
 
+        // If topgrade was run from pwsh, but we are trying to run powershell, then
+        // the inherited PSModulePath breaks module imports
+        if !self.is_pwsh {
+            command.env_remove("PSModulePath");
+        }
+
         Ok(command)
     }
 
@@ -158,7 +153,7 @@ impl Powershell {
             // Find the index of our target policy
             let target_idx = valid_policies.iter().position(|&p| p == policy);
 
-            let mut command = Self::build_command_internal(powershell, "Get-ExecutionPolicy");
+            let mut command = Self::build_command_internal(powershell, self.is_pwsh, "Get-ExecutionPolicy");
 
             let current_policy = command
                 .output_checked_utf8()
@@ -187,7 +182,7 @@ impl Powershell {
         if let Some(powershell) = &self.path {
             let cmd = format!("Get-Module -ListAvailable {}", module_name);
 
-            return Self::build_command_internal(powershell, &cmd)
+            return Self::build_command_internal(powershell, self.is_pwsh, &cmd)
                 .output_checked()
                 .map(|output| !output.stdout.trim_ascii().is_empty())
                 .unwrap_or(false);