A new way to run development builds with a temporary profiles.ini

[Dave Townsend]

When you run a development version of Firefox with `./mach run` it runs with a temporary profile but still uses the profiles.ini file that is shared with all normal versions of Firefox. For the most part this is fine, but if you're testing any features that make modifications to the profiles list (profile management, profile refresh, backup for example) then you run the risk of accidentally breaking your actual profile data, or your profile data gets in the way of what you're trying to test.

You can now work around this. I have landed https://bugzilla.mozilla.org/show_bug.cgi?id=1996224 so now you can run a development build with `./mach run -a` and it will use an entirely separate copy of profiles.ini that you can modify at will without having to worry about your own data.

The temporary database will persist in the same way that the temporary profile does, until something causes the object directory to be clobbered.