Make: remove 'v3.0-beta1-' prefix from "git hash"

`git describe --dirty --always` will produce a hash in the form
`{most recent tag}-{commits since that tag}-{hash}` with a `-dirty` suffix
when there are uncommitted changes. Since the app version and the most
recent tag already overlap, the app version is removed from the git hash
to avoid overly long hashes. They are now of the form `{commits since last
tag}-{hash}`.

Makefile

@@ -5,7 +5,7 @@ PKG := git.blender.org/flamenco
 VERSION := 3.0-beta1
 RELEASE_CYCLE := beta
 
-GITHASH := $(shell git describe --dirty --always)
+GITHASH := $(subst v${VERSION}-,,$(shell git describe --dirty --always))
 LDFLAGS := -X ${PKG}/internal/appinfo.ApplicationVersion=${VERSION} \
 	-X ${PKG}/internal/appinfo.ApplicationGitHash=${GITHASH} \
 	-X ${PKG}/internal/appinfo.ReleaseCycle=${RELEASE_CYCLE}