Add `xpi-snapshot` build target.
Add `xpi-snapshot` build target.

New target automatically creates an unsigned XPI with the SHA1 value of
the current Git HEAD in the file name. This eases the upload process for
manual nightlies in the GitHub repository easier.

downthemall/anticontainer,downthemall/anticontainer,marianocarrazana/anticontainer,marianocarrazana/anticontainer,marianocarrazana/anticontainer,downthemall/anticontainer