Bugfix: cmake: move setting of rpath in front of third party handling

[benegee]

Describe your changes here:

When building and installing t8code's current June release, dynamic linking of libp4est.so.4.0.0 fails because libsc.so.4.0.0 cannot be found. This is caused by a missing rpath setting. In fact, when running make install we see

-- Set non-toolchain portion of runtime path of "/home/bene/install/t8code-current-debug/lib/libp4est.so.4.0.0" to ""

The reason seems to be the order in which the rpaths are set up within CMakeLists.txt. Before introducing FetchContent for third party sources (#2303), it happened before sc and p4est were added, now it is done after.

Resolves #2334

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General

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.57%. Comparing base (4a82a3b) to head (53a1164).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2335   +/-   ##
=======================================
  Coverage   82.57%   82.57%           
=======================================
  Files         116      116           
  Lines       18553    18553           
=======================================
  Hits        15320    15320           
  Misses       3233     3233           

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