feat(checksum): Generate shasum/sha256sum -c compatible sha256 file

Commands shasum -a 256 -c (or) sha256sum -c can read the SHA sum and validate the TAR/ZIP archive
Example:
  Download helm-v3.0.2-darwin-amd64.tar.gz.sha256 and helm-v3.0.2-darwin-amd64.tar.gz and running below will resule in
    shasum -a 256 -c helm-v3.0.2-darwin-amd64.tar.gz.sha256
    helm-v3.0.2-darwin-amd64.tar.gz: OK

Closes #4968

Signed-off-by: Thilak Somasundaram <t2same@gmail.com>

Makefile

@@ -156,7 +156,7 @@ sign:
 .PHONY: checksum
 checksum:
 	for f in _dist/*.{gz,zip} ; do \
-		shasum -a 256 "$${f}"  | awk '{print $$1}' > "$${f}.sha256" ; \
+		shasum -a 256 "$${f}"  > "$${f}.sha256" ; \
 	done
 
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