version: add fallback value for RELEASE_VERSION in Makefile (pingcap#852

)

- Updated the Makefile to ensure `RELEASE_VERSION` always has a valid value.
- Added a fallback to `v9.0.0-alpha` if `git describe --tags --dirty` returns an empty string.
- Ensured compatibility with environments like GitHub Actions where tags might not be fetched.

Makefile

@@ -59,6 +59,9 @@ RELEASE_VERSION =
 ifeq ($(RELEASE_VERSION),)
 	RELEASE_VERSION := $(shell git describe --tags --dirty)
 endif
+ifeq ($(RELEASE_VERSION),)
+	RELEASE_VERSION := v9.0.0-alpha
+endif
 
 # Version LDFLAGS.
 LDFLAGS += -X "$(CDC_PKG)/version.ReleaseVersion=$(RELEASE_VERSION)"