build: add ability to override date in pkgver when building from custom source

Signed-off-by: Vasiliy Doylov <nekocwd@mainlining.org>

docs/environment_variables.md

@@ -26,3 +26,7 @@ password is written to a temporary file and can be read from
 `/proc/<pid>/environ`. Make sure you are aware of the security implications,
 consider using this feature only with test passwords or in environments such as
 live operating systems running in memory.
+
+## `PMB_VER_DATE`
+
+This variable can be used to override date string when building package using `--src`

pmb/build/_package.py

@@ -3,6 +3,7 @@
 import datetime
 import functools
 import operator
+import os
 from typing import Any, TypedDict
 from collections.abc import Callable
 from pmb.build.other import BuildStatus
@@ -115,7 +116,7 @@ def get_pkgver(original_pkgver: str, original_source: bool = False) -> str:
     # Append current date
     no_suffix = original_pkgver.split("_", 1)[0]
     now = datetime.datetime.now()
-    new_suffix = "_p" + now.strftime("%Y%m%d%H%M%S")
+    new_suffix = "_p" + os.getenv("PMB_VER_DATE", now.strftime("%Y%m%d%H%M%S"))
     return no_suffix + new_suffix