Add BB downloading for OpenBLAS (#30497)
* Auto-detect binarybuilder triplet
* Add OpenBLAS BinaryBuilder installation scaffolding
Also make it easier to add more BB-cached versions of dependencies in the future
* Enable `fixup-libgfortran.sh` to directly ask `$FC` for paths
* Tell Appveyor and Travis to use BinaryBuilder OpenBLAS
Also allow the build system to auto-guess the triplet
