Simple fix to the `submit.yml` script to run `apt-get update` prior to installing needed packages on Linux. Without this, the request to install packages can fail to find the requested pacakges due to an out-of-date cache.
Compare the [failing build](https://github.com/kevinrushforth/jfx/actions/runs/480626290) before the fix with the [successful build](https://github.com/kevinrushforth/jfx/actions/runs/480641431) after the fix. Note that this is targeted to `jfx16` so that any subsequent pull request for `jfx16` will be able to have a successful build on Linux. ------------- Commit messages: - 8259639: GitHub actions: build fails on Linux due to missing apt-get update Changes: https://git.openjdk.java.net/jfx/pull/375/files Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=375&range=00 Issue: https://bugs.openjdk.java.net/browse/JDK-8259639 Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod Patch: https://git.openjdk.java.net/jfx/pull/375.diff Fetch: git fetch https://git.openjdk.java.net/jfx pull/375/head:pull/375 PR: https://git.openjdk.java.net/jfx/pull/375
