performance: avoid jinja2 import at startup unless needed (#43237)

`jinja2` can be a costly import, and right now it happens at startup every time we run
Spack. This slows down `spack --print-shell-vars` a bit, which is needed by `setup-env.*sh`.

lib/spack/spack/detection/test.py

@@ -9,8 +9,6 @@
 import tempfile
 from typing import Any, Deque, Dict, Generator, List, NamedTuple, Tuple
 
-import jinja2
-
 from llnl.util import filesystem
 
 import spack.repo
@@ -85,6 +83,8 @@ def _mock_layout(self) -> Generator[List[str], None, None]:
             self.tmpdir.cleanup()
 
     def _create_executable_scripts(self, mock_executables: MockExecutables) -> List[pathlib.Path]:
+        import jinja2
+
         relative_paths = mock_executables.executables
         script = mock_executables.script
         script_template = jinja2.Template("#!/bin/bash\n{{ script }}\n")