Fix dumpvars $PATH / $TMPDIR

Instead of just using the host $PATH during dumpvars, use our path
configuration from the regular build. But instead of creating a ton of
symlinks to the interposer, just use a small directory of symlinks. This
only takes ~3ms (vs ~300ms), at the expense of error logging. Since we
do just about the same product configuration at the start of the build,
we can just rely on the logging there.

This fixes warnings printed by the Mac build, since we using the host
`date` instead of the toybox version:

usage: date [-jnRu] [-d dst] [-r seconds] [-t west] [-v[+|-]val[ymwdHMS]] ...
            [-f fmt date | [[[mm]dd]HH]MM[[cc]yy][.ss]] [+format]
usage: date [-jnRu] [-d dst] [-r seconds] [-t west] [-v[+|-]val[ymwdHMS]] ...
            [-f fmt date | [[[mm]dd]HH]MM[[cc]yy][.ss]] [+format]

TMPDIR was being shared with the main build, but we're allowed to run in
parallel. If a build is started while we're running, our TMPDIR may be
cleared. Instead, create our own temporary directory, which doesn't need
to be a stable value (as we never do caching here).

Fixes: 141893400
Test: (on mac) source build/envsetup.sh; lunch
Change-Id: I0ff536e71dc5649cae26daa0087eb80861704021