Before this change, when a functional test is run without building the source, the error message suggested that previous release binaries were missing.
When no previous release version is set, make the error message more specifically about bitcoind.
To test, try this before and after:
0git clean -dfx
1cmake -B build
2build/test/functional/mining_basic.py
3cmake --build build
4build/test/functional/mining_basic.py
5build/test/functional/wallet_backwards_compatibility.py
6test/get_previous_releases.py
7build/test/functional/wallet_backwards_compatibility.py