Steps to reproduce

Absent multi-value variants on external packages automatically get whatever's requested by spack install or spack spec. This bit me when trying to bootstrap go on an M1 machine using gccgo with an external GCC that in reality had only c/c++/fortran languages.

Information on your system

* **Spack:** 0.17.1-1105-2f7cc643ee
* **Python:** 3.9.9
* **Platform:** darwin-bigsur-m1
* **Concretizer:** clingo

General information

• I have run spack debug report and reported the version of Spack/Python/Platform
• I have searched the issues of this repo and believe this is not a duplicate
• I have run the failing commands in debug mode and reported the output