Remove version pin of Git dependency in Linux build container Dockerfile

A Docker container is used to produce the Linux build of Arduino IDE.

The dependencies of the build are pinned to a specific version in the Dockerfile in order to ensure a stable environment
in the images.

One such dependency is Git. The version of Git available from the package repository of the Ubuntu 18.04 distro used for
the image is too outdated to be used for this purpose. For this reason, a PPA is used as the source for the Git package.
Unfortunately the maintainers of the PPA remove the previous package every time they add a package for a newer version
of Git. This breaks the version pinned installation of the Git package:

5.515 E: Version '1:2.42.0-0ppa1~ubuntu18.04.1' for 'git' was not found

For this reason it is necessary to unpin Git in the Dockerfile. This will cause whichever version is available from the
PPA to be installed each time the image is built. Although not ideal for this application, that shouldn't cause any
problems in practice since Git has quite a stable and carefully maintained interface.

.github/workflows/assets/linux.Dockerfile

@@ -32,7 +32,7 @@ RUN \
   apt-get \
     --yes \
     install \
-      "git=1:2.42.0-0ppa1~ubuntu18.04.1" && \
+      "git" && \
   \
   apt-get \
     --yes \