Fix removing files recursively for bash on z/OS

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What does this MR do?

This MR changes the find -exec terminator used in shells/bash.go. Executing without this change causes this message to appear on the client:

FSUM6506 find: non-terminated "-exec" argument list
Usage: find directory ... expression

And this message to appear on the runner:

time="2025-06-04T10:40:40-04:00" level=warning msg="Error while executing file based variables removal script" error="exit status 1" job=0 project=0

Why was this MR needed?

On Z, the find commands -exec argument cannot be terminated with a + and must use \;. When using the \; terminator the command used, rm -f {}, will be executed once per file/directory instead of combining all the files into a single command.

IE: rm -f file.txt; rm -f file2.txt; VS rm -f file.txt file2.txt

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