Pass OMP_THREAD_LIMIT to Tesseract subprocesses instead of modifying parent env

Instead of setting OMP_THREAD_LIMIT in the parent process's environment,
calculate the thread limit in the validate hook and pass it through to
Tesseract subprocess calls via the env parameter. This avoids polluting
the parent process's environment while still controlling Tesseract's
thread usage.

src/ocrmypdf/subprocess/__init__.py

@@ -23,13 +23,13 @@ from ocrmypdf.exceptions import MissingDependencyError
 log = logging.getLogger(__name__)
 
 Args = Sequence[Path | str]
-OsEnviron = os._Environ  # pylint: disable=protected-access
+Environ = Mapping[str, str] | os._Environ  # pylint: disable=protected-access
 
 
 def run(
     args: Args,
     *,
-    env: OsEnviron | None = None,
+    env: Environ | None = None,
     logs_errors_to_stdout: bool = False,
     check: bool = False,
     **kwargs,
@@ -81,7 +81,7 @@ def run_polling_stderr(
     *,
     callback: Callable[[str], None],
     check: bool = False,
-    env: OsEnviron | None = None,
+    env: Environ | None = None,
     **kwargs,
 ) -> CompletedProcess:
     """Run a process like ``ocrmypdf.subprocess.run``, and poll stderr.
@@ -116,8 +116,8 @@ def run_polling_stderr(
 
 
 def _fix_process_args(
-    args: Args, env: OsEnviron | None, kwargs
-) -> tuple[Args, OsEnviron, logging.Logger, bool]:
+    args: Args, env: Environ | None, kwargs
+) -> tuple[Args, Environ, logging.Logger, bool]:
     if not env:
         env = os.environ
 
@@ -142,7 +142,7 @@ def get_version(
     *,
     version_arg: str = '--version',
     regex=r'(\d+(\.\d+)*)',
-    env: OsEnviron | None = None,
+    env: Environ | None = None,
 ) -> str:
     """Get the version of the specified program.