.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>

.github/workflows/test.yml

@@ -19,6 +19,7 @@ env:
   # toplevel directories "src" (for the checked out source code), and "gomodcache"
   # and other caches as siblings to follow.
   GOMODCACHE: ${{ github.workspace }}/gomodcache
+  CMD_GO_USE_GIT_HASH: "true"
 
 on:
   push:

.github/workflows/vet.yml

@@ -6,6 +6,7 @@ env:
   # toplevel directories "src" (for the checked out source code), and "gomodcache"
   # and other caches as siblings to follow.
   GOMODCACHE: ${{ github.workspace }}/gomodcache
+  CMD_GO_USE_GIT_HASH: "true"
 
 on:
   push: