functions: Persist SOURCE_DATE_EPOCH for downstream jobs

Write the value into output/SOURCE_DATE_EPOCH on the bootstrap-pr build
slave (where core/.git exists) so downstream jobs that consume the
artifacts bundle - and no longer have core/.git - can read the same
value back instead of leaving the variable unset.

Ticket: ENT-14061
Signed-off-by: Lars Erik Wik <lars.erik.wik@northern.tech>

Author: larsewi

build-scripts/functions

@@ -797,10 +797,21 @@ log_error() {
 }
 
 # Pin embedded build timestamps so two builds of the same source produce identical binaries.
-if [ -z "$SOURCE_DATE_EPOCH" ] && [ -d "$BASEDIR/core/.git" ]; then
-    SOURCE_DATE_EPOCH=$(git -C "$BASEDIR/core" log -1 --format=%ct)
-    export SOURCE_DATE_EPOCH
-    log_info "SOURCE_DATE_EPOCH=$SOURCE_DATE_EPOCH"
+# In bootstrap-pr we derive it from core's HEAD commit time and persist it into
+# output/ so downstream jobs (which consume the bootstrap artifacts but no
+# longer have core/.git) can recover the same value.
+if [ -z "$SOURCE_DATE_EPOCH" ]; then
+    if [ -d "$BASEDIR/core/.git" ]; then
+        SOURCE_DATE_EPOCH=$(git -C "$BASEDIR/core" log -1 --format=%ct)
+        mkdir -p "$BASEDIR/output"
+        echo "$SOURCE_DATE_EPOCH" > "$BASEDIR/output/SOURCE_DATE_EPOCH"
+    elif [ -f "$BASEDIR/output/SOURCE_DATE_EPOCH" ]; then
+        SOURCE_DATE_EPOCH=$(cat "$BASEDIR/output/SOURCE_DATE_EPOCH")
+    fi
+    if [ -n "$SOURCE_DATE_EPOCH" ]; then
+        export SOURCE_DATE_EPOCH
+        log_info "SOURCE_DATE_EPOCH=$SOURCE_DATE_EPOCH"
+    fi
 fi
 
 _IS_FUNCTIONS_SOURCED=yes