[rush] Inconsistent yaml serialization during rush update

[dmichon-msft]

Summary

Depending on if the lockfile gets modified by the package manager or not, the version of pnpm-lock.yaml that gets copied back to common/config/rush/pnpm-lock.yaml will have or not have quotes around link: fields.

Repro steps

1. Enable usePnpmPreferFrozenLockfileForRushUpdate
2. Make an unnecessary invocation of rush update. Pnpm will see that the lockfile is up-to-date and not modify it, but rush will copy back the version it emitted.

Solution is to have rush always read and reserialize pnpm-lock.yaml when writing back to common/config/rush/pnpm-lock.yaml instead of copying.

Details

Standard questions

Please answer these questions to help us investigate your issue more quickly:

Question	Answer
@microsoft/rush globally installed version?	5.42.4
rushVersion from rush.json?	5.42.4
useWorkspaces from rush.json?	true
Operating system?	Windows
Would you consider contributing a PR?	Yes
Node.js version (node -v)?	12.22.1