Cabal should respect PKG_CONFIG

**Describe the bug**
Cabal should probably respect the `PKG_CONFIG` environment variable, rather than hard-coding the name "pkg-config", as done in, e.g., [Distribution.Simple.Program.Builtin](https://github.com/haskell/cabal/blob/00a2351789a460700a2567eb5ecc42cca0af913f/Cabal/src/Distribution/Simple/Program/Builtin.hs#L345-L348). For instance, `mingw` packages are moving to use [pkgconf](https://packages.msys2.org/base/mingw-w64-pkgconf), which leads to trouble with Cabal, as `pkg-config` and `pkgconf` cannot both be installed.