x/pkgsite: build issues on AIX and Plan 9

[matloob]

We've disabled the AIX and Plan 9 builders for pkgsite because of two dependencies that break on those platforms respectively:

• github.com/go-git/go-billy/v5 and github.com/go-git/go-git/v5 are broken on AIX (These are the same packages causing x/vulndb: failure to build on aix/ppc64 #58308)
• github.com/lib/pq is broken on Plan 9 (I've sent pull request use build tag to not build ssl_permissions.go on Plan9 lib/pq#1133) but it seems like pull requests are not frequently reviewed.

Once these fixed either upstream or through removing a dependency, we can remove the code in x/build/dashboard/builders.go that disables these on the Plan9 and AIX builders and their misc-compile builders.

[gopherbot]

Change https://go.dev/cl/509475 mentions this issue: dashboard: stop building packagesite on aix and plan9 misc-compile

[jamalc]

@matloob anything left for this issue?

[matloob]

I was thinking that this issue could track the work to eventually reenable the plan9 and aix builders.

[gopherbot]

Change https://go.dev/cl/524935 mentions this issue: dashboard: reenable pkgsite on AIX builders