Update _WalkEachTargetPerFramework to walk TFMs in parallel

[baronfel]

Bug

Fixes: NuGet/Home#13776

Description

This PR changes the MSBuild invocations in this Target to issue parallel build requests for each target where possible. This results in less overhead and more parallelism by default in the build, and in my testing results in a 50% reduction in time taken for this target. This reduction remains consistent as the number of TFMs grows and should be more pronounced as the work done in this
target (i.e. custom user targets) takes more time.

I've included the use of the conventional BuildInParallel property to control this parallelism - if specified the performance is essentially the same as the old version of this target.

Before:

After:

PR Checklist

• Meaningful title, helpful description and a linked NuGet/Home issue
• Added tests (no need, no user facing change)
• Link to an issue or pull request to update docs if this PR changes settings, environment variables, new feature, etc. (no need, doesn't require new docs).

[baronfel]

cc @zivkan per our discussion

[baronfel]

Pinging a few folks for review: @jeffkl @zivkan :)

[zivkan]

waiting for next release, as per label. Maybe we need to update the bot to ignore PRs with the label.

[baronfel]

bot pls. I'm begging you bot.