[CMake] Fix relocatable install patch command with Xcode generator - #6444

Something I would not have never solved by myself 🤖....

At the first build (just after cmake's configuration/generation), XCode (and not ninja) was failing with a weird error on some plugins (just discovered it was with plugins with a dot 👀). But it was compiling fine if you do a second build 🤪

Makefile/Ninja unescape "$$" to "$" before running the shell, Xcode does not. The leftover "$$" made the shell expand ${CMAKE_INSTALL_PREFIX...} itself:
- plugins with a dot in their name (e.g. SoftRobots.Inverse) failed with "bad substitution" on the first build after each configure (set -e in the Xcode script phase defeats the trailing "|| true");
- other plugins got a broken cmake_install.cmake patch (prefix set to "$").

Use a single "$" when the generator is Xcode, as already done for Visual Studio.

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