bpo-45020: Default to using frozen modules unless running from source tree.

[ericsnowcurrently]

Currently the default is "off". Switching it on means users get the benefit without having to do anything. The special-case for running-in-source-tree is so contributors don't get surprised when their stdlib changes don't get used.

https://bugs.python.org/issue45020

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @ericsnowcurrently for commit b12ada3b3249484a8adca22e3ac82be8a1a3fc04 🤖

If you want to schedule another build, you need to add the ":hammer: test-with-buildbots" label again.

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @ericsnowcurrently for commit fc7a72f 🤖

If you want to schedule another build, you need to add the ":hammer: test-with-buildbots" label again.

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @ericsnowcurrently for commit f1d3878 🤖

If you want to schedule another build, you need to add the ":hammer: test-with-buildbots" label again.