os/exec: Plan 9 build has been broken by a Windows security fix (also breaks 1.19.3 and 1.18.8) [1.18 backport]

[gopherbot]

@mdempsky requested issue #56544 to be considered for backport to the next 1.18 minor release.

@gopherbot Please backport to 1.18 and 1.19. The Windows security fix inadvertently broke Plan 9.

[gopherbot]

Change https://go.dev/cl/447875 mentions this issue: [release-branch.go1.18] os/exec: allow NUL in environment variables on Plan 9

[gopherbot]

Closed by merging ca57180 to release-branch.go1.18.