setup/unixstyle-install: fix up only installed ".desktop" files

Closes PR 14354
(cherry picked from commit 1d0e4bedde0d7db5758070fa01147c7c11a59254)
setup/dirs: fix `get-apps-dir`

Affects the location of ".desktop" file installation, especially in
a Unix-style installation.
(cherry picked from commit 7b4ea16bc0a62dfef1c3c0ed0c0acb8c6d2caeb1)
raco setup: fix the way that "stamp" is compiled for avoided files

Instead of claiing "beginning of time" and an empty SHA1, use
`file-stamp-in-paths` to get a proper stamp.

This fixes a problem when installing built packages into user scope
where the built packages refers to an installation-scope library
and SHA1s need to be checked to fix up file timestamps.
(cherry picked from commit 5ada90bfd26568cbdd33989f0b51c61afe7b1bde)