ci: fix sed pattern for capturing PREV_PROVIDER_VERSION in schema-version-diff target at Makefile

previous pattern only handled :=
update the pattern to handle Makefile assignments ?= and =

Signed-off-by: Erhan Cagirici <erhan@upbound.io>

Author: erhancagirici

Makefile

@@ -282,7 +282,7 @@ crddiff:
 
 schema-version-diff:
 	@$(INFO) Checking for native state schema version changes
-	@export PREV_PROVIDER_VERSION=$$(git cat-file -p "${GITHUB_BASE_REF}:Makefile" | sed -nr 's/^export[[:space:]]*TERRAFORM_PROVIDER_VERSION[[:space:]]*:=[[:space:]]*(.+)/\1/p'); \
+	@export PREV_PROVIDER_VERSION=$$(git cat-file -p "${GITHUB_BASE_REF}:Makefile" | sed -nr 's/^export[[:space:]]*TERRAFORM_PROVIDER_VERSION[[:space:]]*[\?:]?=[[:space:]]*(.+)/\1/p'); \
 	echo Detected previous Terraform provider version: $${PREV_PROVIDER_VERSION}; \
 	echo Current Terraform provider version: $${TERRAFORM_PROVIDER_VERSION}; \
 	mkdir -p $(WORK_DIR); \