[Github] Default to non-root user in linux CI container

This patch sets the default user in the linux CI container to a non-root user,
which enables properly testing a couple of features, particularly in
libcxx.

Author: boomanaiden154

.github/workflows/containers/github-action-ci/Dockerfile

@@ -60,3 +60,10 @@ RUN apt-get update && \
 
 ENV LLVM_SYSROOT=$LLVM_SYSROOT
 ENV PATH=${LLVM_SYSROOT}/bin:${PATH}
+
+# Create a new user to avoid test failures related to a lack of expected
+# permissions issues in some tests. Set the user id to 1001 as that is the
+# user id that Github Actions uses to perform the checkout action.
+RUN useradd gha -u 1001 -m -s /bin/bash
+USER gha
+