[SCons] Get git commit from environment variable

In some build environments, particularly conda forge, we use archives of
the Cantera source code downloaded from GitHub. These archives do not
include the .git folder, so we cannot use the git CLI to retrieve the
commit information. This change reads the commit hash from the
CT_GIT_COMMIT environment variable if its set. This will allow us to
bring the output from all our packages closer together.

SConstruct

@@ -162,11 +162,16 @@ cantera_version = "3.1.0a4"
 cantera_pure_version = re.match(r'(\d+\.\d+\.\d+)', cantera_version).group(0)
 cantera_short_version = re.match(r'(\d+\.\d+)', cantera_version).group(0)
 
-try:
-    cantera_git_commit = get_command_output("git", "rev-parse", "--short", "HEAD")
+cantera_git_commit = os.environ.get("CT_GIT_COMMIT")
+if not cantera_git_commit:
+    try:
+        cantera_git_commit = get_command_output("git", "rev-parse", "--short", "HEAD")
+        logger.info(f"Building Cantera from git commit {cantera_git_commit!r}")
+    except (subprocess.CalledProcessError, FileNotFoundError):
+        cantera_git_commit = "unknown"
+else:
     logger.info(f"Building Cantera from git commit {cantera_git_commit!r}")
-except (subprocess.CalledProcessError, FileNotFoundError):
-    cantera_git_commit = "unknown"
+
 
 # Python Package Settings
 python_min_version = parse_version("3.8")