tool/gocross: fix environment variable clearing in gocross-wrapper.ps1

The -Environment argument to Start-Process is essentially being treated
as a delta; removing a particular variable from the argument's hash
table does not indicate to delete. Instead we must set the value of each
unwanted variable to $null.

Updates https://github.com/tailscale/corp/issues/29940

Signed-off-by: Aaron Klotz <aaron@tailscale.com>

tool/gocross/gocross-wrapper.ps1

@@ -176,10 +176,13 @@ $bootstrapScriptBlock = {
     if (-not $gocrossOk) {
         $goBuildEnv = Copy-Environment
         $goBuildEnv['CGO_ENABLED'] = '0'
-        $goBuildEnv.Remove('GOOS')
+        # Start-Process's -Environment arg applies diffs, so instead of removing
-        $goBuildEnv.Remove('GOARCH')
+        # these variables from $goBuildEnv, we must set them to $null to indicate
-        $goBuildEnv.Remove('GO111MODULE')
+        # that they should be cleared.
-        $goBuildEnv.Remove('GOROOT')
+        $goBuildEnv['GOOS'] = $null
+        $goBuildEnv['GOARCH'] = $null
+        $goBuildEnv['GO111MODULE'] = $null
+        $goBuildEnv['GOROOT'] = $null
 
         $procExe = Join-Path $toolchain 'bin' 'go.exe' -Resolve
         $proc = Start-Process -FilePath $procExe -WorkingDirectory $repoRoot -Environment $goBuildEnv -ArgumentList 'build', '-o', $gocrossPath, "-ldflags=-X=tailscale.com/version.gitCommitStamp=$wantVer", 'tailscale.com/tool/gocross' -NoNewWindow -Wait -PassThru
@@ -195,7 +198,10 @@ Start-ChildScope -ScriptBlock $bootstrapScriptBlock
 $repoRoot = Get-RepoRoot
 
 $execEnv = Copy-Environment
-$execEnv.Remove('GOROOT')
+# Start-Process's -Environment arg applies diffs, so instead of removing
+# these variables from $execEnv, we must set them to $null to indicate
+# that they should be cleared.
+$execEnv['GOROOT'] = $null
 
 $argList = Copy-ScriptArgs