x/pkgsite-metrics: build failures on `aix` and `solaris` since CL 464635

[bcmills]

CL 464635 (attn @julieqiu, @jba) added a dependency on github.com/go-git/go-billy, which seems to rely on the not-quite-portable flock system call. It fails to build on both the aix and the solaris builders:

https://build.golang.org/log/16e55aaaf5cd0530197e39196e8b5574ce26d1d5

aix-ppc64 at fcd0e0963f3bdf289216c1083f1439a71dc571a4 building pkgsite-metrics at 4260eec4c420289d41ba5777be826919d16b4408
…
# github.com/go-git/go-billy/v5/osfs
../../../../pkg/mod/github.com/go-git/go-billy/v5@v5.4.0/osfs/os_posix.go:15:14: undefined: unix.Flock
../../../../pkg/mod/github.com/go-git/go-billy/v5@v5.4.0/osfs/os_posix.go:22:14: undefined: unix.Flock
?   	golang.org/x/pkgsite-metrics/cmd/worker	[no test files]
?   	golang.org/x/pkgsite-metrics/internal/derrors	[no test files]
?   	golang.org/x/pkgsite-metrics/internal/observe	[no test files]
?   	golang.org/x/pkgsite-metrics/internal/proxy/proxytest	[no test files]
?   	golang.org/x/pkgsite-metrics/internal/testing/testhelper	[no test files]
go version devel fcd0e0963f3bdf289216c1083f1439a71dc571a4 aix/ppc64
Checking go and sh files for license header
go vet -all ./...

# github.com/go-git/go-billy/v5/osfs
../../../../pkg/mod/github.com/go-git/go-billy/v5@v5.4.0/osfs/os_posix.go:15:14: undefined: unix.Flock
../../../../pkg/mod/github.com/go-git/go-billy/v5@v5.4.0/osfs/os_posix.go:22:14: undefined: unix.Flock
command failed
Installing: github.com/client9/misspell/cmd/misspell
⋯
FAILED; see errors above
--- FAIL: Test (41.70s)
    all_test.go:26: exit status 1
FAIL
FAIL	golang.org/x/pkgsite-metrics	41.754s

https://build.golang.org/log/1cef20b2732f80f0dc5839208b565552e0c9d0ab

solaris-amd64-oraclerel at 01a5a83cfbe594538f6fe9f49bd27d9dc36155a4 building pkgsite-metrics at 4260eec4c420289d41ba5777be826919d16b4408
…
# golang.org/x/pkgsite-metrics/internal/stdlib.test
/opt/golang/tmp/workdir-host-solaris-oracle-amd64-oraclerel/go/pkg/tool/solaris_amd64/link: running gcc failed: exit status 1
Undefined			first referenced
 symbol  			    in file
flock                               /opt/golang/tmp/workdir-host-solaris-oracle-amd64-oraclerel/tmp/go-link-1706862252/go.o
ld: fatal: symbol referencing errors
collect2: error: ld returned 1 exit status
…
FAIL	golang.org/x/pkgsite-metrics/internal/stdlib [build failed]

[bcmills]

This can be addressed by either fixing the build on those platforms or, if x/pkgsite-metrics intentionally does not support them, by configuring cmd/coordinator not to test that repo on them.

[julieqiu]

Fixed in https://go-review.git.corp.google.com/c/pkgsite-metrics/+/465217