RFR: 8306543: GHA: MSVC installation is failing when component is already installed

[Christoph Langer]

I did some more investigation for this issue.

The real root cause for the failing install seems to be that the option --installPath nowadays needs a correct Windows path with backslashes. This probably must have been different beforehand. So I corrected that.

However, as we figured that seemingly the correct toolchain could (and is) already installed by default, it makes sense to skip the expensive installation operation which takes some 10+ minutes in that case.

I also tested the scenario with an older toolchain version which would need installing, e.g. 14.28.

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Commit messages:
 - JDK-8306543

Changes: https://git.openjdk.org/jdk/pull/13574/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13574&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8306543
  Stats: 15 lines in 1 file changed: 14 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/13574.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13574/head:pull/13574

PR: https://git.openjdk.org/jdk/pull/13574