[jfx16] RFR: 8259639: GitHub actions: build fails on Linux due to missing apt-get update

[Kevin Rushforth]

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 Without this, the request to install packages can fail to find the requested pacakges due to an out-of-date cache.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.

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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