Fix Coverage loading a custom config file (#10670)

PathGlob glob param expects a collection (iterable) of strings, passing it a string prevents it from working properly.

[ci skip-rust]

src/python/pants/backend/python/rules/coverage.py

@@ -170,7 +170,7 @@ async def create_coverage_config(coverage: CoverageSubsystem) -> CoverageConfig:
         config_contents = await Get(
             DigestContents,
             PathGlobs(
-                globs=coverage.config,
+                globs=tuple(coverage.config,),
                 glob_match_error_behavior=GlobMatchErrorBehavior.error,
                 description_of_origin=f"the option `--{coverage.options_scope}-config`",
             ),