v0.15.4 (2020-08-12)

This release contains one feature addition:

• Users can set SPACK_GNUPGHOME to override Spack's GPG path (#17139)

Several bugfixes for CUDA, binary packaging, and spack -V:

• CUDA package's .libs method searches for libcudart instead of libcuda (#18000)
• Don't set CUDAHOSTCXX in environments that contain CUDA (#17826)
• buildcache create: NoOverwriteException is a warning, not an error (#17832)
• Fix spack buildcache list --allarch (#17884)
• spack -V works with releases/latest tag and shallow clones (#17884)

And fixes for GitHub Actions and tests to ensure that CI passes on the
release branch (#15687, #17279, #17328, #17377, #17732).