Bug 1798779 - Stop using double-colon rules in client.mk. r=firefox-build-system-reviewers,nalexander

Since bug 1784179, we make `make` keep going after errors. When an error
happens, `make` stops rules at the command that fails, but if there are
multiple double-colon rules for the same target, it still runs the
remaining rules, which has undesirable side effects in client.mk: it
stops an already stopped sccache, which leaves an error in the log.

Differential Revision: https://phabricator.services.mozilla.com/D161083