RFR: CODETOOLS-7902987: Git build should record the actual tag, not the latest one

[Aleksey Shipilev]

Use `git describe` to poll the actual tag. Otherwise JTReg build would always report the latest tag (currently `jtreg6`), regardless of the actually built version. See more details in the bug report.

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Commit messages:
 - CODETOOLS-7902987: Git build should record the actual tag, not the latest one

Changes: https://git.openjdk.java.net/jtreg/pull/14/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jtreg&pr=14&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-7902987
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jtreg/pull/14.diff
  Fetch: git fetch https://git.openjdk.java.net/jtreg pull/14/head:pull/14

PR: https://git.openjdk.java.net/jtreg/pull/14