Steps to reproduce

When two different operating systems have the same spack installation the spack compiler command can handle this correctly, but not the spack external find command.
So it is not possible to use external libraries in conjunction with different operating systems even when setting the operating system explicitly in .spack/packages.yaml

Error message

No explicit error message, as spack does not spec out this correclty

Information on your system

I saw this issue when trying to comparing a sles-12-sp4 with a sles-15-sp3