ci: fix Windows benchmarks running all tests instead of just benchmarks

The -run "^$" flag was being mangled by cmd.exe's argument processing.
The ^ character is cmd.exe's escape character, so go.cmd's cmd.exe layer
eats it, turning -run "^$" into -run "$" which matches all test names.
This caused the benchmark job to run every test, leading to timeouts
and Go runtime crashes.

Use -run XXXXNothingXXXX instead, which avoids special characters
entirely.

Updates #19252

Change-Id: I888c124254dd2767a40b61bcd68dbc9b22ad35a1
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

.github/workflows/test.yml

@@ -273,7 +273,10 @@ jobs:
       # Don't use -bench=. -benchtime=1x.
       # Somewhere in the layers (powershell?)
       # the equals signs cause great confusion.
-      run: ./tool/go test ./... -bench . -benchtime 1x -run "^$"
+      # Don't use -run "^$" 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
       env:
         NOPWSHDEBUG: "true" # to quiet tool/gocross/gocross-wrapper.ps1 in CI