if MPSD_MICROARCH is not defined, use archspec to determine

What does the MR provide?

• If MPSD_MICROARCH is not defined as a environment variable, then use archspec to determine it.
• if archspec is not available, ask user to provide it via error message and stop.

Where does this matter?

• script creates a folder for the microarchitecture.
• Found in Docker container that this would be dev-23a/UNKNOWN_MICROARCH/
• the same will happen on other user machines (unless they are the Linux laptops provided by MPSD; but we know several people who prefer to run their own distribution, or even non-linux system (i.e. Mac).

mpsd-software-environment.py

@@ -519,7+519,7 @@
             else:
                 raise ValueError(
                     f"Some error occurred when calling 'archspec cpu': {process=}"
                 )
 
     # at this point, we have determined the microarchitecture
     log_metadata("microarchitecture", microarch)
@@ -572,7+572,7 @@
     # Set required variables
     release_base_dir = script_dir / mpsd_release
     os.environ.get("MPSD_OS", "UNKNOWN_OS")
-    microarch = get_native_microarchitecture()
-    toolchain_dir = release_base_dir / microarch
+    mpsd_microarch = os.environ.get("MPSD_MICROARCH", "UNKNOWN_MICROARCH")
+    toolchain_dir = release_base_dir / mpsd_microarch
     toolchain_dir.mkdir(parents=True, exist_ok=True)
     spack_setup_script = release_base_dir / "spack-environments" / "spack_setup.sh"
     install_flags = []