Verify that GODEBUG=gocachehash=1 output from ./tool/go includes the
git revision from go.toolchain.rev, ensuring that bumping the Tailscale
Go fork (without a Go version number change) properly invalidates the
build cache.
The test only runs in CI or when the current Go binary is the Tailscale
toolchain (GOROOT contains /.cache/tsgo/), so open source contributors
using stock Go aren't forced to download tsgo.
Fixestailscale/corp#36589
Change-Id: Ia98d3a3aa8c7fa67f9a0293066fa02a1997dcb95
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
