Pin fmt version at 8.0.1 (facebook#196)

Summary:
Change build script to pin fmt version to same as folly's to minimize future breaks.

Pull Request resolved: facebook#196

Test Plan:
Built successfully on a fresh clone of CacheLib. (Also had to change `external_git_branch=dev` for zstd to deal with the cmake/zstd issue in facebook#194, but that should resolve when gets merged into release)

**Context:** OSS build broke between 3-5 Jan 2023, likely due to changes in folly. While switching to v9.1.0 or 9.0.0 fixes the issue at hand, it seems sensible to match folly, which specifies fmt v8.0.1: https://github.com/facebook/folly/blob/main/build/fbcode_builder/manifests/fmt

> facebook#62 agordon: For the other packages, you'll notice we do use a specific git tag or branch… I notice `fmt` is an exception - not pinned to a specific git tag or revision - likely an omission that can be fixed.

Related CacheLib issues: facebook#186, facebook#189, facebook#107, facebook#97, facebook#62
Possibly related CacheLib commit: 67cc11a

Last working (Jan 3): https://github.com/facebook/CacheLib/actions/runs/3826992478
First failed (Jan 5): https://github.com/facebook/CacheLib/actions/runs/3844002307/jobs/6546742348
Error: `error: static assertion failed: Cannot format an argument. To make type T formattable provide a formatter<T> specialization: https://fmt.dev/latest/api.html#udt`

Reviewed By: therealgymmy

Differential Revision: D43517927

Pulled By: jiayuebao

fbshipit-source-id: 2d28791f7804d862b646263b96b10b835f843d8c

contrib/build-package.sh

@@ -160,6 +160,7 @@ case "$1" in
     REPODIR=cachelib/external/$NAME
     SRCDIR=$REPODIR
     external_git_clone=yes
+    external_git_tag="8.0.1"
     cmake_custom_params="-DBUILD_SHARED_LIBS=ON"
     if test "$build_tests" = "yes" ; then
         cmake_custom_params="$cmake_custom_params -DFMT_TEST=YES"