Auto merge of #20998 - matt-y:python-virtualenv-setup-pip-incovation-fix, r=jdm

Change to pip dependency install invocation during virtualenv setup in build

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Fix for https://github.com/servo/servo/issues/20990

In short, the pip invocation was causing my build on OSX to break. Using `python -m pip` in place of `pip` when setting up the virtual environment fixes the environment setup failure and allows the build to proceed.

---
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #20990

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- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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python/mach_bootstrap.py

@@ -193,18 +193,7 @@ def _activate_virtualenv(topdir, is_firefox):
     if need_pip_upgrade:
         # Upgrade pip when virtualenv is created to fix the issue
         # https://github.com/servo/servo/issues/11074
-        if sys.platform in ['msys', 'win32']:
-            python = _get_exec_path(PYTHON_NAMES, is_valid_path=check_exec_path)
-            if not python:
-                sys.exit("Python is either not installed or not found in virtualenv.")
-
-            _process_exec([python, "-m", "pip", "install", "-I", "-U", "pip"])
-        else:
-            pip = _get_exec_path(PIP_NAMES, is_valid_path=check_exec_path)
-            if not pip:
-                sys.exit("Python pip is either not installed or not found in virtualenv.")
-
-            _process_exec([pip, "install", "-I", "-U", "pip"])
+        _process_exec([python, "-m", "pip", "install", "-I", "-U", "pip"])
 
     for req_rel_path in requirements_paths:
         req_path = os.path.join(topdir, req_rel_path)
@@ -217,11 +206,7 @@ def _activate_virtualenv(topdir, is_firefox):
         except OSError:
             pass
 
-        pip = _get_exec_path(PIP_NAMES, is_valid_path=check_exec_path)
-        if not pip:
-            sys.exit("Python pip is either not installed or not found in virtualenv.")
-
-        _process_exec([pip, "install", "-I", "-r", req_path])
+        _process_exec([python, "-m", "pip", "install", "-I", "-r", req_path])
 
         open(marker_path, 'w').close()