Bug 1693723 - Avoid bootstrapping when an explicit path is given for tools....

Bug 1693723 - Avoid bootstrapping when an explicit path is given for tools. r=firefox-build-system-reviewers,mhentges

This changes things such that setting e.g. NASM=/usr/bin/nasm will avoid
bootstrapping nasm even when bootstrapping is enabled.

This is not applied to CC/CXX/HOST_CC/HOST_CXX because things are more
complicated.

This also simplifies how check_prog is called for a bootstrapped tool,
and avoids the repetition of when.

CBINDGEN handling needs the pattern being applied manually because it
currently doesn't use check_prog. Once --enable-bootstrap=install
becomes the default on developer builds, it will be possible to simplify
this.

Differential Revision: https://phabricator.services.mozilla.com/D105718