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/step.rs

@@ -464,7 +464,7 @@ impl Step {
             Pnpm => runner.execute(*self, "pnpm", || node::run_pnpm_upgrade(ctx))?,
             Poetry => runner.execute(*self, "Poetry", || generic::run_poetry(ctx))?,
             Powershell => {
-                let powershell = powershell::Powershell::new();
+                let powershell = ctx.powershell();
                 if powershell.is_available() {
                     runner.execute(Powershell, "Powershell Modules Update", || {
                         powershell.update_modules(ctx)