setup: take a post version from environment variable

Make it possible to add a post version (per PEP-440) to the current agent
version at build time. This will be useful because we want to continuously
build the agent on CI and avoid version conflicts when publishing to test pypi.
So when returning the agent version add anything that has been passed
in ELASTIC_CI_POST_VERSION environment variable as post version.

Refs #1994

setup.py

@@ -101,9 +101,9 @@ def get_version():
         if line.startswith("__version__"):
             version_tuple = ast.literal_eval(line.split(" = ")[1])
             version_str = ".".join(map(str, version_tuple))
-            local_version = os.getenv("ELASTIC_CI_LOCAL_VERSION")
-            if local_version:
-                return f"{version_str}-dev{local_version}"
+            post_version = os.getenv("ELASTIC_CI_POST_VERSION")
+            if post_version:
+                return f"{version_str}.post{post_version}"
             return version_str
     return "unknown"