github: Properly propagate BUILD_DATE to BUILD_DATE_UNIX

Previously, when echoing the definition of BUILD_DATE to
$GITHUB_OUTPUT, that doesn't make it available as an environment
variable. (And if it would be echoed to $GITHUB_ENV, it still
wouldn't take effect until the next step within the same job either.)

Therefore, this previously set BUILD_DATE to $(date -d "" +%s),
which seems to produce a timestamp corresponding to the start of
the current day.

.github/workflows/build.yml

@@ -43,7 +43,8 @@ jobs:
           fi
           echo TAG=$TAG >> $GITHUB_OUTPUT
           echo COMMIT_DATE_UNIX=$(git log -1 --pretty=%ct $GITHUB_SHA) >> $GITHUB_OUTPUT
-          echo BUILD_DATE=$(date -u '+%FT%TZ') >> $GITHUB_OUTPUT
+          BUILD_DATE=$(date -u '+%FT%TZ')
+          echo BUILD_DATE=${BUILD_DATE} >> $GITHUB_OUTPUT
           echo BUILD_DATE_UNIX=$(date -d "${BUILD_DATE}" +%s) >> $GITHUB_OUTPUT
           cat $GITHUB_OUTPUT
           cat $GITHUB_OUTPUT >> parameters.txt