tool: replace go.cmd with a 19KB Rust go.exe wrapper

go.cmd used cmd.exe to invoke PowerShell, which mangled arguments:
cmd.exe treats ^ as an escape character (so -run "^$" became -run "$",
running all tests instead of none) and = signs also caused issues in
the PowerShell→cmd.exe argument passing layer.

Replace it with a tiny no_std Rust binary (19KB, 32-bit x86 for
universal Windows compat: x86/x64/ARM64) that directly invokes the
Tailscale Go toolchain via CreateProcessW. The raw command line from
GetCommandLineW is passed through to CreateProcessW with only argv[0]
replaced, so arguments are never parsed or re-escaped.

The binary also handles first-run toolchain download natively using
curl.exe and tar.exe (both ship with Windows 10+), so PowerShell is
no longer required for normal operation. The PowerShell fallback is
only used for the rare TS_USE_GOCROSS=1 path.

PowerShell prefers go.exe over go.cmd when resolving ./tool/go, so
this is a drop-in replacement.

With go.exe in place, the CI can use the natural -bench=. -benchtime=1x
-run="^$" flags directly.

Also removes tool/go-win.ps1 which is now unused.

Updates #19255

Change-Id: I80da23285b74796e7694b89cff29a9fa0eaa6281
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

.github/workflows/test.yml

@@ -270,13 +270,7 @@ jobs:
     - name: bench all
       if: matrix.key == 'win-bench'
       working-directory: src
-      # Don't use -bench=. -benchtime=1x.
-      # Somewhere in the layers (powershell?)
-      # the equals signs cause great confusion.
-      # Don't use -run "^$" either; the ^ is cmd.exe's escape
-      # character, so go.cmd's cmd.exe layer eats it, turning
-      # -run "^$" into -run "$" which matches all test names.
-      run: ./tool/go test ./... -bench . -benchtime 1x -run XXXXNothingXXXX
+      run: ./tool/go test ./... -bench=. -benchtime=1x -run="^$"
       env:
         NOPWSHDEBUG: "true" # to quiet tool/gocross/gocross-wrapper.ps1 in CI