RFR: 8257924: Use full path when running external executable from jpackage

[Alexander Matveev]

Changed all external executables to run via full path to avoid potential issues.

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Commit messages:
 - 8257924: Use full path when running external executable from jpackage

Changes: https://git.openjdk.java.net/jdk/pull/1710/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1710&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257924
  Stats: 22 lines in 9 files changed: 1 ins; 0 del; 21 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1710.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1710/head:pull/1710

PR: https://git.openjdk.java.net/jdk/pull/1710