Look for command next to kleopatra or gpg

If the program to run is given as plain name (i.e. neither as absolute
path nor as relative path with an actual '/' or '\') then look for the
program first next to the GnuPG programs (i.e. in the installation path
of GnuPG reported by gpgme) and then next to kleopatra. This ensures that
executables that are installed next to gpg or kleopatra are preferred
over executables found in PATH.

(cherry picked from commit d6cbd15561f999584f951d919fdc7037a5a57cab)

• GnuPG-bug-id: T8082