crypto/x509: respect GODEBUG changes during program lifetime [1.19 backport]

[gopherbot]

@rsc requested issue #56436 to be considered for backport to the next 1.19 minor release.

@gopherbot please backport

[gopherbot]

Change https://go.dev/cl/445655 mentions this issue: [release-branch.go1.19] crypto/x509: respect GODEBUG changes for allowing SHA1 certificates

[gopherbot]

Closed by merging 6109c07 to release-branch.go1.19.