3.14.1.1 and 3.12.1.0 build failure: Installation directories are not prefix_relative: internal error InstallDirs.libsubdir

[i0ntempest]

Describe the bug
Build failure with numerous of errors like this:

  Failed to build base16-bytestring-1.0.2.0. The failure occurred during the
  configure step.
  Build log (
  /opt/local/var/macports/build/_Users_runner_work_macports-ports_macports-ports_ports_lang_cabal/cabal/work/.home/.cabal/logs/ghc-9.6.6/bs16-bytstrng-1.0.2.0-7247b4c4.log
  ):
  Configuring library for base16-bytestring-1.0.2.0...
  Error: [Cabal-6000]
  Installation directories are not prefix_relative:
  internal error InstallDirs.libsubdir
  CallStack (from HasCallStack):
    error, called at src/Distribution/Simple/InstallDirs.hs:146:19 in Cabal-3.14.1.1-inplace:Distribution.Simple.InstallDirs

See CI logs here for complete build logs

To Reproduce
Steps to reproduce the behavior:
Basically trying to build cabal 3.14.1.1 or 3.12.1.0 from source, using official cabal binary

Expected behavior
It builds

System information

• macOS 15
• cabal 3.14.1.1 or 3.12.1.0, using ghc 9.12.1 and 9.6.6

Additional context
Could be somehow related to 7b90583 but I'm not sure.
Build args:
--project-file=cabal.release.project --bindir=/opt/local/bin --datadir=/opt/local/share/cabal
.cabal/config file:

relocatable: True
install-method: copy
installdir: /opt/local/bin

install-dirs global
  prefix: /opt/local
  bindir: /opt/local/bin
  libdir: /opt/local/lib
  libsubdir: cabal
  dynlibdir: /opt/local/lib
  libexecdir: /opt/local/libexec
  libexecsubdir: cabal
  datadir: /opt/local/share/cabal
  docdir: /opt/local/share/doc/cabal
  htmldir: /opt/local/share/doc/cabal
  haddockdir: $htmldir
  sysconfdir: /opt/local/etc/cabal

program-locations
  gcc-location: /usr/bin/clang