[jdk17u-dev] RFR: 8303066: SOURCE_DATE_EPOCH is set by default also in older versions of JDK

[KIRIYAMA Takuya]

I removed SetupReproducibleBuild. But, at building ct.sym, SOURCE_DATE_EPOCH environment variable is needed. So I added setting routine if SOURCE_DATE_EPOCH is not set.
Would you please review this fix?

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Commit messages:
 - 8303066: SOURCE_DATE_EPOCH is set by default also in older versions of JDK

Changes: https://git.openjdk.org/jdk17u-dev/pull/1186/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=1186&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303066
  Stats: 28 lines in 3 files changed: 4 ins; 21 del; 3 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/1186.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev pull/1186/head:pull/1186

PR: https://git.openjdk.org/jdk17u-dev/pull/1186