.github/workflows: set CMD_GO_USE_GIT_HASH=true for our cmd/go

Updates tailscale/go#150
Updates tailscale/corp#28679

Change-Id: Ieb4780f157451f5c6660c96c6efaec9ddcfcb415
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f