Do not define project(CBMC ...) twice to fix CMake failures

[tautschnig]

project(P, ...) sets the CMake variablee P_SOURCE_DIR to whichever
directory that CMake file is in. We happen to rely on the value of
CBMC_SOURCE_DIR in several places. Invoking project(P, ...) twice
in different directories for the same value of P will cause
hard-to-reproduce behaviour. Even though this duplication was in place
ever since, 7949cac, we apparently got lucky until a few days ago.
Now, however, we see sporadic failures on GitHub runners, such as
https://github.com/diffblue/cbmc/actions/runs/10677152028/job/29591809621.
GitHub's runner image notes do not point out any recent change to CMake,
but it might as well be changes to the kernel's scheduler.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.34%. Comparing base (27b845c) to head (aeeb39f).
Report is 2 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8435   +/-   ##
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  Coverage    78.34%   78.34%           
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  Files         1726     1726           
  Lines       188769   188769           
  Branches     18503    18503           
========================================
  Hits        147891   147891           
  Misses       40878    40878           

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[TGWDB]

Approving as a suitable fix for now. Reading it there seems to be some tension between CBMC_SOURCE_DIR and SRC neither of which are where the source code lives. Maybe disambiguating with CBMC_ROOT_DIR might be clearer... but this is an improvement and not necessary for now.