build-package.sh: never use git outside of package build process.

termux-packages is not guaranteed to be always used as git repository.
Relying on git here may lead to issues.

P.S. Avoid unfinished work on master branch!

build-package.sh

@@ -3,7 +3,7 @@
 
 set -e -o pipefail -u
 
-SOURCE_DATE_EPOCH=$(git log -1 --pretty=%ct)
+SOURCE_DATE_EPOCH=$(date "+%s")
 export SOURCE_DATE_EPOCH
 
 : "${TMPDIR:=/tmp}"