I generated a Signet address on a Bitcoin Core PR branch and obtained some Signet from the faucet. How can I locate that private key? I previously (months ago) generated a Signet address on a Bitcoin Core PR branch and obtained some Signet from the faucet to that address. How can I locate that private key in my file system if I know the Bitcoin address? How can I locate that private key if I have forgotten the Bitcoin address?
As Pieter says in the comment the private key is in the wallet.dat file and you can use:
./src/bitcoin-cli -signet dumpprivkey insert_signet_address
to get the private key for that signet address.
There are separate wallet.dat files for each chain (mainnet, testnet, signet, regtest) that persist across the different PR branches and different versions of Bitcoin Core you run. These aren't ever replaced or written over. If you have signet Bitcoin on one PR branch, then you build and run another PR branch you will still have access to that same signet Bitcoin that you had on the previous branch in the usual way.
Pieter added below:

Every chain has its own distinct default datadir, and everything in
that datadir (including blocks, chainstate, peers.dat, onion
addresses/keys, ban data, fee estimates, mempool dumps, ...) is shared
between all codebases that use that chain.