Silence warnings about non-existent index for mirrors that don't have and don't need a buildcache index

[climbfuji]

Description

lib/spack/spack/binary_distribution.py: silence warnings about non-existent index for mirrors, because this warning is issued for all mirrors (source, bootstrap, ...), even if they don't have or don't need a build cache index to function correctly. See comments from Spack developers just below my changes.

These warnings look like legitimate errors, clutter the output of automated build/deploy systems, and confuse users and developers.

I do not indent to send this upstream but maintain the diff in our fork/branch.

Testing

See JCSDA/spack-stack#1896

[mathomp4]

@climbfuji Is this related to the:

==> Warning: The following issues were ignored while updating the indices of binary caches
  Multiple errors during fetching:
        Error 1: BuildcacheIndexNotExists: Index not found in cache file:///swbuild/gmao_SIteam/spack-stack/build-cache/v3/manifests/index/index.manifest.json
        Error 2: BuildcacheIndexNotExists: Index not found in cache file:///swbuild/gmao_SIteam/spack-stack/build-cache/build_cache/index.json

things I get?

[climbfuji]

@climbfuji Is this related to the:

==> Warning: The following issues were ignored while updating the indices of binary caches
  Multiple errors during fetching:
        Error 1: BuildcacheIndexNotExists: Index not found in cache file:///swbuild/gmao_SIteam/spack-stack/build-cache/v3/manifests/index/index.manifest.json
        Error 2: BuildcacheIndexNotExists: Index not found in cache file:///swbuild/gmao_SIteam/spack-stack/build-cache/build_cache/index.json

things I get?

Exactly. These get silenced by the PR.