Check for existence of .bin dir in pnp dependencies before adding to PATH.

[hWorblehat]

Summary

Avoids #6683 in many situations by siginificantly reducing the length that the PATH is increased by.
This issue could still raise its head in some situations, if the user already has a PATH whose length is close to the Windows character limit.

Test plan

• Ran yarn build && yarn test - all succeeded
• Tested newly-built yarn against my own repository that was previously exhibiting "'node' is not recognized as an internal or external command" when using Yarn PnP on Windows #6683. Lifecycle scripts now work.

[buildsize]

File name	Previous Size	New Size	Change
yarn-[version].noarch.rpm	1.1 MB	1.1 MB	-19 bytes (0%)
yarn-[version].js	4.47 MB	4.47 MB	96 bytes (0%)
yarn-legacy-[version].js	4.66 MB	4.66 MB	94 bytes (0%)
yarn-v[version].tar.gz	1.12 MB	1.11 MB	-1.85 KB (0%)
yarn_[version]all.deb	814.75 KB	814.11 KB	-654 bytes (0%)

[arcanis]

Thanks! Will wait for the tests to finish and merge it