Install pip-requirements-parser in workspace

and not in the virtualenv.

action/main.py

@@ -22,25 +22,27 @@
     print(f"::error::Working directory does not exist: {WORKING_DIRECTORY}", flush=True)
     sys.exit(21)
 
-run([sys.executable, "-m", "venv", str(ENV_PATH)], check=True)  # nosec
 
-def pip_install(*packages):
-    proc = run(  # nosec
-        [str(ENV_BIN / "python"), "-m", "pip", "install", *packages],
+def pip_install(*packages, *, python):
+    pip_proc = run(  # nosec
+        [str(python), "-m", "pip", "install", *packages],
         check=False,
         stdout=PIPE,
         stderr=STDOUT,
         encoding="utf-8",
     )
-    print(proc.stdout, end="")
-    if proc.returncode:
+    print(pip_proc.stdout, end="")
+    if pip_proc.returncode:
         print(f"::error::Failed to install {' '.join(packages)}.", flush=True)
-        sys.exit(proc.returncode)
+        sys.exit(pip_proc.returncode)
+
 
-pip_install("pip-requirements-parser")
+pip_install("pip-requirements-parser", python=sys.executable)
 
 from pip_requirements_parser import parse_reqparts_from_string
 
+run([sys.executable, "-m", "venv", str(ENV_PATH)], check=True)  # nosec
+
 req = ["graylint[color]"]
 if VERSION:
     if VERSION.startswith("@"):
@@ -62,7 +64,7 @@ def pip_install(*packages):
     req.append(str(linter_requirement))
     linter_options.extend(["--lint", linter])
 
-pip_install(*req)
+pip_install(*req, python=ENV_BIN / "python")
 
 
 base_cmd = [str(ENV_BIN / "darker")]