runtime: race condition raised with parallel tests, panic(nil) and -race [1.21 backport]

@dr2chase requested issue #64649 to be considered for backport to the next 1.21 minor release.

> @gopherbot please consider this for backport to 1.21.  It broke someone's integration tests, and since `GODEBUG=panicnil` (or go.mod version = 1.20) is a compatibility feature added to 1.21, it is a regression, if you look at it right.
