message: Unbreak fmt upgrades
diff: @@ -4,7 +4,13 @@ option(VCPKG_DEPENDENCY_EXTERNAL_FMT "Use an external version of the fmt library
  # builds which have restricted internet access; see azure-pipelines/signing.yml
  # Note that the SHA512 is the same, so vcpkg-tool contributors need not be concerned that we built
  # with different content.
  -set(VCPKG_FMT_URL "https://github.com/fmtlib/fmt/archive/refs/tags/9.1.0.tar.gz" CACHE STRING "URL to the fmt release tarball to use.")
  +# A cache variable cannot be used it here because it will break contributors' builds on fmt update.
  +if("$CACHE{VCPKG_FMT_URL}" MATCHES "^https://github.com/fmtlib/fmt/archive/refs/tags")
  + unset(VCPKG_FMT_URL CACHE) # Fix upgrade
  +endif()
  +if(NOT VCPKG_FMT_URL)
  + set(VCPKG_FMT_URL "https://github.com/fmtlib/fmt/archive/refs/tags/9.1.0.tar.gz")
  +endif()
  include(FetchContent)
  FetchContent_Declare(
  