tool/gocross: replace use of Start-Process -Wait flag with WaitForExit

-Wait does not just wait for the created process; it waits for the
entire process tree rooted at that process! This can cause the shell
to wait indefinitely if something in that tree fired up any background
processes.

Instead we call WaitForExit on the returned process.

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

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

tool/gocross/gocross-wrapper.ps1

@@ -190,7 +190,8 @@ $bootstrapScriptBlock = {
         $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
+        $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 -PassThru
+        $proc.WaitForExit()
         if ($proc.ExitCode -ne 0) {
             throw 'error building gocross'
         }
@@ -222,10 +223,12 @@ if ($Env:TS_USE_GOCROSS -ne '1') {
     }
 
     $procExe = Join-Path $toolchain 'bin' 'go.exe' -Resolve
-    $proc = Start-Process -FilePath $procExe -WorkingDirectory $repoRoot -Environment $execEnv -ArgumentList $argList -NoNewWindow -Wait -PassThru
+    $proc = Start-Process -FilePath $procExe -WorkingDirectory $repoRoot -Environment $execEnv -ArgumentList $argList -NoNewWindow -PassThru
+    $proc.WaitForExit()
     exit $proc.ExitCode
 }
 
 $procExe = Join-Path $repoRoot 'gocross.exe' -Resolve
-$proc = Start-Process -FilePath $procExe -WorkingDirectory $repoRoot -Environment $execEnv -ArgumentList $argList -NoNewWindow -Wait -PassThru
+$proc = Start-Process -FilePath $procExe -WorkingDirectory $repoRoot -Environment $execEnv -ArgumentList $argList -NoNewWindow -PassThru
+$proc.WaitForExit()
 exit $proc.ExitCode