Disable switching to build-agent user on launch

This commit disables automatically switching to the `build-agent`,
a non-root user, context on launch because doing so can corrupt the
permissions of the files created inside the GitHub Actions runner.

This should be kept disabled until a fundamental solution to this
problem is provided by the GitHub, or a usable workaround is found.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>

Dockerfile

@@ -57,4 +57,6 @@ RUN groupadd -g $GID -o build-agent && \
     echo 'build-agent ALL = NOPASSWD: ALL' > /etc/sudoers.d/build-agent && \
     chmod 0440 /etc/sudoers.d/build-agent
 
-USER build-agent
+# NOTE: Do not switch to a non-root user because this creates all sorts of
+#       permission-related problems with the GitHub Actions runner.
+# USER build-agent