Get rid of --dirty

For some reason building changes `Cargo.lock` which adds `-dirty` to the end of the package name of subsequent because `git describe` thinks the `librespot` working directory has changed.

For example:
`raspotify_0.31.15-2-gd716a8a~librespot.v0.3.1-72-g6c2491b-dirty_amd64.deb`

Instead of:

`raspotify_0.31.15-2-gd716a8a~librespot.v0.3.1-72-g6c2491b_amd64.deb`

build.sh

@@ -20,7 +20,7 @@ fi
 
 # Get the git rev of librespot for .deb versioning
 cd librespot
-LIBRESPOT_GIT_VER="$(git describe --tags --always --dirty 2>/dev/null || echo unknown)"
+LIBRESPOT_GIT_VER="$(git describe --tags --always 2>/dev/null || echo unknown)"
 
 # Build librespot
 cargo build --release --target $BUILD_TARGET --no-default-features --features "alsa-backend pulseaudio-backend"