.github/workflows: clarify tailcat breakage check is only a heads-up

Rename the tailcat job added in 6c06e00ad to
heads-up-hypothetical-tailcat-build-using-this-commit and print an
explanation on failure: the job builds tailcat against tailscale.com at
this commit rather than the version tailcat pins, so a failure doesn't
break tailcat until it bumps its dependency. It only matters if the API
or behavior breakage is unexpected. People were misreading failures as
something more urgent.

Updates tailscale/corp#24454

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: I3f8a1c92d47e60b5a2e9c14f7d08b361
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