Some commands aren't resistent to missing source directory of pinned packages

[kit-ty-kate]

As presented in #6597, opam pin list currently fails in the following manner when the source directory isn't present:

$ opam install .
[...]
Would you like to install? [Y/n] n
$ opam pin
/usr/local/bin/opam: "chdir" failed on .../.opam-switch/sources/<pkg>: No such file or directory

I suspect other opam commands also fail in a similar manner while trying to get the git revision or similar info.

This specific chdir issue might be better fixed in the future by 69b4478 from #5966 (related to #3171, #2442, #969, …)

[reynir]

I stumbled upon this just now. It seemed fixed itself after I ran opam update. At least I can not reproduce any longer.