allow macOS installer builds to package pre-built html docs (GH-20715)

allow macOS installer builds to package pre-built html docs (GH-20715)

build-installer now looks in its directory of source tarballs
for a suitable html tarball of the same version.  If so, it
will unpack and use it rather than rebuilding the html format
documentation set from the source repo.  This is intended as
a speedup for test builds of the installer.  Files names must
be in the same format as produced by the docs build for download,
for example, `python-3.9.0b1-docs-html.tar.bz2`.