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

[gopherbot]

@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.

[gopherbot]

Change https://go.dev/cl/550236 mentions this issue: [release-branch.go1.21] runtime: add race annotations in IncNonDefault

[cagedmantis]

Approved pending a small additional CL.

[dr2chase]

Small additional CL was merged into CL 550236

[gopherbot]

Closed by merging 8c6078a to release-branch.go1.21.