Replace virtualenv with Python's built-in venv (#30377)

* Replace virtualenv with Python's built-in venv.

* Apply Delan's suggestions and make a couple small fixes

- Fix a tidy warning about directories that don't exist
- Use shutil instead of the redundant get_exec_path
- Miscellaneous cleanups

* Fix typo in environment variable

* fix bug where pip still tries to the wrong site-packages

---------

Co-authored-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Delan Azabani <dazabani@igalia.com>

README.md

@@ -25,18 +25,17 @@ manually, try the [manual build setup][manual-build].
 - Install [Xcode](https://developer.apple.com/xcode/)
 - Install [Homebrew](https://brew.sh/)
 - Run `curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh`
-- Run `pip3 install virtualenv`
 - Run `./mach bootstrap`<br/>
   *Note: This will install the recommended version of GStreamer globally on your system.*
 
 ### Linux
 
 - Run `curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh`
-- Install Python and virtualenv
-    - **Debian-like:** Run `sudo apt install python3-virtualenv python3-pip`
-    - **Fedora:** Run `sudo dnf install python3 python3-virtualenv python3-pip python3-devel`
-    - **Arch:** Run `sudo pacman -S --needed python python-virtualenv python-pip`
-    - **Gentoo:** Run `sudo emerge dev-python/virtualenv dev-python/pip`
+- Install Python
+    - **Debian-like:** Run `sudo apt install python3-pip`
+    - **Fedora:** Run `sudo dnf install python3 python3-pip python3-devel`
+    - **Arch:** Run `sudo pacman -S --needed python python-pip`
+    - **Gentoo:** Run `sudo emerge dev-python/pip`
 - Run `./mach bootstrap`
 
 ### Windows