feat(ci): Add sync-source-mirrors job after successful x86 build

[MatthewWEdwards]

Prerequisites

• [x] I have read the Contributing Guidelines.
• [x] I have read the Code of Conduct.
• [x] If this is a non-trivial change, I have already opened an accompanying Issue.
• [x] If applicable, I have included documentation updates alongside my code changes.

Description

Add downloaded tarballs to the shared source_mirrors filesystem on self-hosted runners. Also add BB_GENERATE_MIRROR_TARBALLS = "1" to bitbake configuration by default for all meta-layers so that tarballs are generated.

Some parts of the terragraph-image-x86.yml workflow are reformatted, and the file is renamed to remove the word "build"

Test Plan

1. Delete facebook/openr GitHub tarball from source mirrors
2. Clear openr build artifacts from sstate-cache
3. Run workflow: https://github.com/terragraph/meta-terragraph/actions/runs/2794158940
4. See new openr tarball in source_mirrors:

ubuntu@ip-10-0-20-186:~/source_mirrors$ ls -lh | grep openr -A 2 -B 2
-rw-r--r-- 1 ubuntu ubuntu  6.7M May 10 18:49 git2_github.com.facebook.fbzmq.git.tar.gz
-rw-r--r-- 1 ubuntu ubuntu   52M May 10 18:49 git2_github.com.facebook.folly.git.tar.gz
-rw-rw-r-- 1 ubuntu ubuntu   30M Aug  4 05:18 git2_github.com.facebook.openr.git.tar.gz
-rw-r--r-- 1 ubuntu ubuntu   21M May 10 18:49 git2_github.com.facebook.proxygen.git.tar.gz
-rw-r--r-- 1 ubuntu ubuntu  6.9M May 10 18:49 git2_github.com.facebook.wangle.git.tar.gz

[MatthewWEdwards]

• Add new configuration file setting BB_GENERATE_MIRROR_TARBALLS to 1
• Rename "downloads" artifact to "sources"
• Add -v flag to cp