Patch for getting EXIFTOOL path from environment

[Carmelo DrRaw]

In order to be able to use a custom exiftool command from inside an AppImage bundle, I had to apply the following patch in order to provide the path to the EXIFTOOL through an environment variable: https://github.com/aferrero2707/art-appimage/blob/master/ci/art-exiftool-base-dir-from-env.patch

Maybe you can consider to incorporate it in the official code?

Thanks!

[agriggio]

metadata: allow to set the base dir of exiftool with an env var (ART_EXIFTOOL_BASE_DIR)

see #30 -- thanks to Carmelo DrRaw for the patch

→ <<cset 3007feba3fce>>

[agriggio]

pushed, thanks!