[webkitpy] Check if directory is writeable before installing required libraries

[akath20]

f0d1f173ed82f586a55ee7e9fa8bff2c372f322f

[webkitpy] Check if directory is writeable before installing required libraries
rdar://123836656

Reviewed by NOBODY (OOPS!).

If tooling is mounted in a read-only location, it should instead install tooling at ~/Library/webkitpy

* Tools/Scripts/webkitpy/__init__.py:

https://github.com/WebKit/WebKit/commit/f0d1f173ed82f586a55ee7e9fa8bff2c372f322f

Misc	iOS, tvOS & watchOS	macOS	Linux	Windows
✅ 🧪 style	✅ 🛠 ios	✅ 🛠 mac	✅ 🛠 wpe	✅ 🛠 wincairo
✅ 🧪 bindings	✅ 🛠 ios-sim	✅ 🛠 mac-AS-debug	✅ 🧪 wpe-wk2
✅ 🧪 webkitperl	✅ 🧪 ios-wk2	✅ 🧪 api-mac	✅ 🧪 api-wpe
✅ 🧪 webkitpy	✅ 🧪 ios-wk2-wpt	✅ 🧪 mac-wk1	✅ 🛠 gtk
	✅ 🧪 api-ios	✅ 🧪 mac-wk2	✅ 🧪 gtk-wk2
	✅ 🛠 tv	✅ 🧪 mac-AS-debug-wk2	❌ 🧪 api-gtk
	✅ 🛠 tv-sim
	✅ 🛠 watch
	✅ 🛠 watch-sim

[webkit-early-warning-system]

EWS run on current version of this PR (hash https://github.com/WebKit/WebKit/commit/f0d1f173ed82f586a55ee7e9fa8bff2c372f322f)

Misc	iOS, tvOS & watchOS	macOS	Linux	Windows
✅ 🧪 style	✅ 🛠 ios	✅ 🛠 mac	✅ 🛠 wpe	✅ 🛠 wincairo
✅ 🧪 bindings	✅ 🛠 ios-sim	✅ 🛠 mac-AS-debug	✅ 🧪 wpe-wk2
✅ 🧪 webkitperl	✅ 🧪 ios-wk2	✅ 🧪 api-mac	✅ 🧪 api-wpe
✅ 🧪 webkitpy	✅ 🧪 ios-wk2-wpt	✅ 🧪 mac-wk1	✅ 🛠 gtk
	✅ 🧪 api-ios	✅ 🧪 mac-wk2	✅ 🧪 gtk-wk2
	✅ 🛠 tv	✅ 🧪 mac-AS-debug-wk2	❌ 🧪 api-gtk
	✅ 🛠 tv-sim
❌ 🛠 🧪 merge	✅ 🛠 watch
✅ 🛠 🧪 unsafe-merge	✅ 🛠 watch-sim

[webkit-commit-queue]

Committed 275564@main (8865cdb1ab93): https://commits.webkit.org/275564@main

Reviewed commits have been landed. Closing PR #25300 and removing active labels.