extending example for buildcaches (#22504)
extending example for buildcaches (#22504)

* extending example for buildcaches

I was attempting to create a local build cache from a directory, and I found the
docs for both buildcaches and mirrors, but did not connect the docs that the
url variable could be the local filesystem variable. I am extending the docs for
buildcaches with an example of creating and interacting with one on the filesystem
because I suspect other users will run into this need and possibly not find what
they are looking for.

Signed-off-by: vsoch <7cb4235d215b6aa84109596e3788aeb2b4ed1c44@users.noreply.github.com>

* adding as follows to spack mirror list

Co-authored-by: Tamara Dahlgren <c6cbf61916fccc63bb27e34a67a98a380db93a98@users.noreply.github.com>

Co-authored-by: vsoch <7cb4235d215b6aa84109596e3788aeb2b4ed1c44@users.noreply.github.com>
Co-authored-by: Tamara Dahlgren <c6cbf61916fccc63bb27e34a67a98a380db93a98@users.noreply.github.com>
LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack