Unset GMOCK_LIB before find_library to force lookup

[ThomsonTan]

Changes

find_library is used to get the path of gmock, and build type (CMAKE_BUILD_TYPE) is checked to link to either gmockd.lib for debug build or gmock.lib for others.

The problem is that find_library stores the found library in cache variable which will NOT be re-checked for the CMake re-run, so if the re-run changes build type, it will link to the previous gmock, which causes runtime exception, as the OTLP test and gmock then run with 2 different flavors of CRT library, and trigger exception when passing C++ types around, such as std::string.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

Merging #726 (a9ff04d) into main (2d2aa71) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #726   +/-   ##
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  Coverage   94.76%   94.76%           
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  Files         217      217           
  Lines        9929     9929           
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  Hits         9409     9409           
  Misses        520      520