spack ci rebuild: Don't install in a login shell (#31771)

On some systems the shell in login mode wipes important parts of the
environment, such as PATH. This causes the build to fail since it can't
find `spack`.

For better robustness, don't use a login shell.

In a full CI job the final spack install is run in an environment formed by scripts running in this order:

export AWS_SECRET=...                  # 1. Load environment from GitLab project variables
source spack/share/spack/setup-env.sh  # 2. Load Spack into the environment (PATH)
spack env activate -V concrete_env     # 3. Activate the concrete environment
source /etc/profile                    # 4. Bash login shell (from -l)
spack install ...
Whereas when a user launches their own container with (docker|podman) run -it, they end up running spack install in an environment formed in this order:

source /etc/bash.bashrc                # (not 4). Bash interactive shell (default with TTY)
export AWS_SECRET=...                  #~1. Manually load environment from GitLab project variables
source spack/share/spack/setup-env.sh  # 2. Load Spack into the environment (PATH)
spack env activate -V concrete_env     # 3. Activate the concrete environment
spack install ...
The big problem being that (4) has a completely different position and content (on Leap 15 and possibly other containers).

So in context, this PR removes (4) from the CI job case, leaving us with the simpler:

export AWS_SECRET=...                  # 1. Load environment from GitLab project variables
source spack/share/spack/setup-env.sh  # 2. Load Spack into the environment (PATH)
spack env activate -V concrete_env     # 3. Activate the concrete environment
spack install ...

lib/spack/spack/cmd/ci.py

@@ -521,10 +521,9 @@ def ci_rebuild(args):
     install_copy_path = os.path.join(repro_dir, "install.sh")
     shutil.copyfile("install.sh", install_copy_path)
 
-    # Run the generated install.sh shell script as if it were being run in
-    # a login shell.
+    # Run the generated install.sh shell script
     try:
-        install_process = subprocess.Popen(["bash", "-l", "./install.sh"])
+        install_process = subprocess.Popen(["bash", "./install.sh"])
         install_process.wait()
         install_exit_code = install_process.returncode
     except (ValueError, subprocess.CalledProcessError, OSError) as inst: