sg: PipeOutputUnbuffered breaks when command exits too quickly

Created by: mrnugget

I had the wrong asdf version so the yarn command would immediately fail here:

$ go run . start web-standalone

💡 Installing 2 commands...

caddy installed
web-standalone-http installed
✅ 2/2 commands installed  █████████████████████████████████████████████████████████████████████████████████████

✅ Everything installed! Booting up the system!

Running caddy...
panic: failed to pipe output: read |0: file already closed

goroutine 179 [running]:
github.com/sourcegraph/sourcegraph/lib/process.PipeOutputUnbuffered.func1({0x6234180, 0xc0000a01b0}, {0x6234580, 0xc000132290})
        /Users/thorstenball/work/sourcegraph/lib/process/pipe.go:32 +0x94
github.com/sourcegraph/sourcegraph/lib/process.pipeOutputWithCopy.func2({0x6234180, 0xc0000a01b0}, {0x6234580, 0xc000132290})
        /Users/thorstenball/work/sourcegraph/lib/process/pipe.go:63 +0x74
created by github.com/sourcegraph/sourcegraph/lib/process.pipeOutputWithCopy
        /Users/thorstenball/work/sourcegraph/lib/process/pipe.go:67 +0x27d
exit status 2

Pretty sure it's related to this: https://github.com/sourcegraph/sourcegraph/pull/30672