Allow to override build date with SOURCE_DATE_EPOCH (#2202)

to make builds reproducible.
See https://reproducible-builds.org/ for why this is good
and https://reproducible-builds.org/specs/source-date-epoch/
for the definition of this variable.

This patch was done while working on reproducible builds for openSUSE.

---------

Co-authored-by: extrawurst <[email protected]>

Author: bmwiedemann

Diff for: CHANGELOG.md

@@ -12,6 +12,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * respect configuration for remote when fetching (also applies to pulling) [[@cruessler](https://github.com/cruessler)] ([#1093](https://github.com/extrawurst/gitui/issues/1093))
 * add `:` character to sign-off trailer to comply with Conventinoal Commits standard [@semioticrobotic](https://github.com/semioticrobotic) ([#2196](https://github.com/extrawurst/gitui/issues/2196))
 
+### Added
+* support overriding `build_date` for [reproducible builds](https://reproducible-builds.org/) [[@bmwiedemann](https://github.com/bmwiedemann)] ([#2202](https://github.com/extrawurst/gitui/pull/2202))
+
 ## [0.26.0+1] - 2024-04-14
 
 **0.26.1**

Diff for: build.rs

@@ -1,3 +1,5 @@
+use chrono::TimeZone;
+
 fn get_git_hash() -> String {
 	use std::process::Command;
 
@@ -18,7 +20,13 @@ fn get_git_hash() -> String {
 }
 
 fn main() {
-	let build_date = chrono::Local::now().date_naive();
+	let now = match std::env::var("SOURCE_DATE_EPOCH") {
+		Ok(val) => chrono::Local
+			.timestamp_opt(val.parse::<i64>().unwrap(), 0)
+			.unwrap(),
+		Err(_) => chrono::Local::now(),
+	};
+	let build_date = now.date_naive();
 
 	let build_name = if std::env::var("GITUI_RELEASE").is_ok() {
 		format!(