[8.x](backport #5610) Handle BUILDKITE_MESSAGE length in pre-command.ps1

[mergify]

Description of problem:
Windows unit test pre-command steps were failing in CI when the BUILDKITE_MESSAGE param was too long, which would occur when commits had very long descriptions. This sometimes happens from automated processes, such as dependency updates. The BUILDKITE_MESSAGE would be very long, which would fill the Windows env var buffer, preventing further pre-command steps from adding directories to the PATH.
Problem report: https://elastic.slack.com/archives/C0522G6FBNE/p1727193548224399

Fix:
Trim the BUILDKITE_MESSAGE param to the INTERNET_MAX_URL_LENGTH of 2048.
This was tested in draft PR #5609 with very long and short commit messages. I confirmed that the Windows unit test steps got past the pre-command phase.

References:

• https://devblogs.microsoft.com/oldnewthing/20100203-00/?p=15083
• https://devblogs.microsoft.com/oldnewthing/20031210-00/?p=41553
• Example of path shortening in shell APIs: https://learn.microsoft.com/en-us/windows/win32/api/shlwapi/nf-shlwapi-pathcreatefromurlw

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  This is an automatic backport of pull request Handle BUILDKITE_MESSAGE length in pre-command.ps1 #5610 done by Mergify.

[elastic-sonarqube]

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