RFR: JDK-8268185: Update GitHub Actions for jtreg 6

[Jonathan Gibbons]

Please review changes to the GitHub Actions for the repo, to use jtreg 6.

There are three small parts to the change:

1. The version info is updated in `make/conf/test-dependencies`
2. The new `make/build.sh` script is used to build `jtreg`, instead of the earlier `build-all.sh` script
3. The format of the tag in the jtreg repo is changed to follow OpenJDK conventions.

Tested implicitly with GitHub Actions on my fork, seen here:
https://github.com/jonathan-gibbons/jdk/actions/runs/903574304

-------------

Commit messages:
 - use bash for build.sh
 - JDK-8268185: Update GitHub Actions for jtreg 6

Changes: https://git.openjdk.java.net/jdk/pull/4343/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4343&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8268185
  Stats: 4 lines in 2 files changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4343.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4343/head:pull/4343

PR: https://git.openjdk.java.net/jdk/pull/4343