Allow to override build date with SOURCE_DATE_EPOCH

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.
Also use UTC to be independent of timezone.

tui/build.rs

@@ -1,7 +1,14 @@
+use chrono::{TimeZone, Utc};
+use std::env;
+
 fn main() {
     // Add current date as a variable to be displayed in the 'Linux Toolbox' text.
+    let now = match env::var("SOURCE_DATE_EPOCH") {
+        Ok(val) => { Utc.timestamp_opt(val.parse::<i64>().unwrap(), 0).unwrap() }
+        Err(_) => Utc::now(),
+    };
     println!(
         "cargo:rustc-env=BUILD_DATE={}",
-        chrono::Local::now().format("%Y-%m-%d")
+        now.format("%Y-%m-%d")
     );
 }