cmd/dist: runPending doesn't JSON-encode skips in -json mode

[dmitshur]

runPending needs to print valid JSON to w.out when dist is invoked in -json mode. It normally does this because it runs a go test -json command, except we missed that in the case it prints a skip message. This causes consumers of dist test -json output to encounter non-JSON in such cases, and making sense of that can be wasteful of human time.

The fix is fairly trivial, but the diff ended up a bit bigger than I'd like as a result of cleaning things up and writing down more of how work fields are used. So I'll split most of that into a separate CL for Go 1.22, and do a minimal safe fix for Go 1.21.

CC @golang/release, @aclements.

[gopherbot]

Change https://go.dev/cl/512719 mentions this issue: cmd/dist: handle -json flag in runPending (minimal)

[gopherbot]

Change https://go.dev/cl/512115 mentions this issue: cmd/dist: handle -json flag in runPending (clean up)

[gopherbot]

Change https://go.dev/cl/513761 mentions this issue: [release-branch.go1.21] cmd/dist: handle -json flag in runPending (minimal)

[gopherbot]

Closed by merging 6df6e61 to release-branch.go1.21.