Change build metadata separator from - to +

SemVer 2.0.0 requires a plus for build metadata.

https://semver.org/
`Build metadata MAY be denoted by appending a plus sign and a series of
dot separated identifiers immediately following the patch or pre-release
version. Identifiers MUST comprise only ASCII alphanumerics and hyphen
[0-9A-Za-z-]. Identifiers MUST NOT be empty. Build metadata SHOULD be
ignored when determining version precedence. Thus two versions that
differ only in the build metadata, have the same precedence. Examples:
1.0.0-alpha+001, 1.0.0+20130313144700, 1.0.0-beta+exp.sha.5114f85.`

build_scripts/append_to_version_file.py

@@ -17,5 +17,5 @@ def set_version_string(new_version_string):
 
 if __name__ == "__main__":
     current_version = get_version_string()
-    new_version = current_version + "-" + argv[1]
+    new_version = current_version + "+" + argv[1]
     set_version_string(new_version)