runtime: use-after-free of `allpSnapshot` in `findRunnable` [1.24 backport]

[gopherbot]

@prattmic requested issue #74414 to be considered for backport to the next 1.24 minor release.

@gopherbot Please backport to 1.23 and 1.24. This is a long-standing bug, but can cause random crashes when increasing GOMAXPROCS.

[gopherbot]

Change https://go.dev/cl/685056 mentions this issue: [release-branch.go1.24] runtime: stash allpSnapshot on the M