fix #858 RunCommand: find server script by CLI; report path in error message

[wti]

Fixes #858.

• Uses ProcessInfo argument 0 (always available) to derive the install dir
• Includes the missing path in the error message
• Suggests make clean before install instead of vice-versa

[wti]

wrt failing test "Site published when Markdown content contains invalid lastModified date", there may be a filesystem race.

The test writes a file and then tries to detect it. This also failed on my machine initially on a clean tree. I tried changing the file-write to be atomic, which worked, but the test also worked after changing it back (I haven't investigated the test cleanup, but it seems to be skipped on failure). I can think of no way that the RunCommand changes trigger this failure.

I'm not set up to replicate CI, but next steps on the test could be to do atomic writes and always clean up (using something like defer{ cleanup... }).