On PR only run partial sync

[ChuckJonas]

As we start to track more and more metadata, our pipeline is going to slow down.

In theory, we could generate a package.xml from only the inbound changes and then use that to sync master from production.

We could have a nightly process (4am) that runs to do a full sync. OR, if we wanted to get fancy, we could have the pipeline check for when the last full sync was, and run a full sync if it's > X days.

Either way, we will need to modify the build package script to look something like this:

git checkout $pr-branch
git merge master
sfdx git:package -d dist/pgk
git checkout master
sfdx force:source:retrieve -x dist/pgk/package.xml
git add .
# I guess if there are ANY changes then we could commit to master and abort?  
# Or would it make more sense to commit / merge master into $pr-branch again 
# and see if there are conflicts?  
# I guess there could be changes that will merge?