Fixing some issue introduced in 4e2b66e9.

The script run_scripts was used to generate multiple bundles (for the
different locales) in parallel. However now that we only have one bundle
for all locales, using run_scripts is not useful anymore, and we can
simplify the build script by removing the use of run_scripts.

As the xz version on the host can be different and produce different
output, we compress the firefox source tarball inside a container.

At the same time we use xz's --threads option to make compression
faster by using multiple threads.

We also move the src-tarballs step definition from rbm.conf to
projects/firefox/config since it's not used in any other project.

We need it for the migration wizard (e.g., to import bookmarks).

This step is needed to set Mullvad Browser as a default browser.

Also, we use the same way as Firefox to identify different installs
uniquely based on the hash of their installation directory.
Therefore, we need to build the CityHash NSIS plugin on Firefox and
consume it in browser.
We do the same for the ApplicationID plugin.

As of March 1st, foursquare.com renewed its certificate and domain
fronting stopped working through this front according to fastly's new
policy: https://github.com/net4people/bbs/issues/309

Use the bspatch file we create during the build to unsign the apk (which
was signed by the QA key) before signing it with the release key.

This build fixes a crash on Android and has a couple of other Android
improvements.

Currently, we sign the APKs that were already signed with the QA key.
However, this makes it impossible to copy our signature on top of APKs
built independently, which would be the last step to reproduce our
builds.

With this commit, we provide a patch to go back to the unsigned (but
already aligned) APK, so that we can keep shipping QA-signed APKs for
testers, but we can go back to the unsigned APK for the final signing.

The changes to apply this patch in the signing scripts will be part of
another commit.

Fixes a bug introduced while resolving #40569.

This commits make alpha and nightlies macOS build include the channel
in the app bundle.

Also, it renames the Project_Name_Channel to display_name to make it
less ambiguous, and now Project_Name_Channel includes the channel also
for the release target, even though it will be rarely used.

The installer for Mullvad Browser is going to be different from Tor
Browser's, because it will offer both a traditional installation
(at the moment only for the current user, to avoid enabling privilege
elevation code paths, but still with a control panel entry) and
portable "installations".

The traditional one will support also setting MB as the default
browser and file associations. However, this is quite tricky, and
having a base installer to start implementing the feature is helpful.
Thus this commit, even though the final installer will be likely
changed.

We will need this plugin for the Mullvad Browser uninstaller.

In this way we can shave a few additional MBs, and get further from the
APK size limit.

Revert until rbm#40072 is fixed.

This reverts commit cc41e2d7.

The IO::CaptureOutput perl module is deprecated, so we switch to
Capture::Tiny. We also remove some uses of IO::CaptureOutput where it
was not needed.