CMake: append should be used for CMAKE_MODULE_PATH

[CraigHutchinson]

Description of Change(s)

CMAKE_MODULE_PATH is a directory list and shall be appended via list(APPEND

Note

No major change but ensures when CMAKE_MODULE_PATH has been defined with alternative defaults on the command-line or via use of CMAKE_PROJECT_TOP_LEVEL_INCLUDES

• I have verified that all unit tests pass with the proposed changes
• [x ] I have submitted a signed Contributor License Agreement

PR Note:

• First commit to confirm CLA
• This change is small enough and deemed self-explanatory to not affect tests, I will however look into executing tests as a sanity check soon but do not see this PR being blocked on this?

[tallytalwar]

Filed as internal issue #USD-8013