Bug 1707030 - Return to having NSDISTMODE=copy impact INSTALL....

Bug 1707030 - Return to having NSDISTMODE=copy impact INSTALL. r=firefox-build-system-reviewers,mhentges

This reverts [part of Bug
1255485](https://hg.mozilla.org/mozilla-central/rev/5888578c22f8dc006cfd6a405c9fc53b2df73b44).

It should have been reverted in Bug 1641291, which allowed
NSDISTMODE=copy to impact build manifests.  This is useful when
cross-compiling: this stops some symlinks within the object directory
itself, say from `dist/include` to an object directory.

Differential Revision: https://phabricator.services.mozilla.com/D113149