Fix startup failure if the cache directory points to a broken symlink

[Earlopain]

Discovered in Shopify/ruby-lsp-rails#278

If the cache directory is a symlink and points to another directory that doesn't exist, saving the cache will fall into an infinite loop. This is because of the retry logic at

bootsnap/lib/bootsnap/load_path_cache/store.rb

Lines 107 to 108 in 8394834

	rescue Errno::EEXIST
	retry

This patch forces a different error by trying to resolve the symlink. Calling File.realpath on a broken symlink will raise Errno::ENOENT which will not be retried.

A setup explanation to even end up in this state is provided at Shopify/ruby-lsp-rails#278 (comment)

@mttkay can you check if this fixes the issue for you?

[mttkay]

I applied the patch locally and I can confirm this fixes the problem for me.

Thank you!