feat: add source cache path attribute for mounting in e.g. containers (#33)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* New Features
* Added an optional configuration to specify a source cache path during
environment initialization, enabling cached sources for improved reuse.
Existing behavior remains unchanged if not provided.
* Tests
* Expanded test coverage to include scenarios using the new source cache
path, ensuring correct handling and integration in typical workflows.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: johanneskoester

snakemake_interface_software_deployment_plugins/__init__.py

@@ -169,6 +169,7 @@ def __init__(
         settings: Optional[SoftwareDeploymentSettingsBase],
         shell_executable: ShellExecutable,
         tempdir: Path,
+        source_cache: Path,
         cache_prefix: Path,
         deployment_prefix: Path,
         pinfile_prefix: Path,
@@ -178,6 +179,7 @@ def __init__(
         self.settings: Optional[SoftwareDeploymentSettingsBase] = settings
         self.shell_executable = shell_executable
         self.tempdir = tempdir
+        self.source_cache: Path = source_cache
         self._deployment_prefix: Path = deployment_prefix
         self._cache_prefix: Path = cache_prefix
         self._pinfile_prefix: Path = pinfile_prefix

snakemake_interface_software_deployment_plugins/tests.py

@@ -152,17 +152,20 @@ def _get_env(self, tmp_path) -> EnvBase:
         deployment_prefix = tmp_path / "deployments"
         cache_prefix = tmp_path / "cache"
         pinfile_prefix = tmp_path / "pinfiles"
+        source_cache = tmp_path / "source_cache"
         tempdir.mkdir(parents=True, exist_ok=True)
         deployment_prefix.mkdir(parents=True, exist_ok=True)
         cache_prefix.mkdir(parents=True, exist_ok=True)
         pinfile_prefix.mkdir(parents=True, exist_ok=True)
+        source_cache.mkdir(parents=True, exist_ok=True)
 
         return env_cls(
             spec=spec,
             within=None,
             settings=self.get_settings(),
             shell_executable=self.shell_executable,
             tempdir=tempdir,
+            source_cache=source_cache,
             deployment_prefix=deployment_prefix,
             cache_prefix=cache_prefix,
             pinfile_prefix=pinfile_prefix,