[release-branch.go1.21] cmd/dist: handle -json flag in runPending (minimal)

[release-branch.go1.21] cmd/dist: handle -json flag in runPending (minimal)

The -json flag is new to Go 1.21, but missed skips in runPending.
This CL adds minimal code to fix that. CL 512115 cleans up a bit.

For #37486.
Fixes #61557.

Change-Id: I53e426c9a5585b2703f0ff6661a0470e1993f960
Reviewed-on: https://go-review.googlesource.com/c/go/+/513761
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>