fix(ci): correct GITHUB_SHA fallback condition (STREQUAL not NOT)

The previous fallback used 'if(NOT TRANSCRIBE_BUILD_COMMIT ...)', but the
variable is initialized to "unknown" (a non-empty string = truthy in CMake),
so 'NOT <var>' was always FALSE and the GITHUB_SHA/GITHUB_REF_NAME fallback
never ran — Linux .so build-id still showed 'unknown unknown'. Use
'STREQUAL "unknown"' instead, matching llama.cpp-omni's build-info.cmake.

Verified: in a no-.git directory with GITHUB_SHA set, the capture now falls
back to the short SHA (commit abc123d, branch main).

Author: Edward Chen

CMakeLists.txt

@@ -96,10 +96,10 @@ endif()
 # provenance instead of "unknown". GITHUB_SHA is the full 40-char commit; trim
 # it to the short form so it matches the `git rev-parse --short HEAD` shape
 # captured above.
-if(NOT TRANSCRIBE_BUILD_COMMIT AND DEFINED ENV{GITHUB_SHA})
+if(TRANSCRIBE_BUILD_COMMIT STREQUAL "unknown" AND DEFINED ENV{GITHUB_SHA})
     string(SUBSTRING "$ENV{GITHUB_SHA}" 0 7 TRANSCRIBE_BUILD_COMMIT)
 endif()
-if(NOT TRANSCRIBE_BUILD_BRANCH AND DEFINED ENV{GITHUB_REF_NAME})
+if(TRANSCRIBE_BUILD_BRANCH STREQUAL "unknown" AND DEFINED ENV{GITHUB_REF_NAME})
     set(TRANSCRIBE_BUILD_BRANCH "$ENV{GITHUB_REF_NAME}")
 endif()
 string(TIMESTAMP TRANSCRIBE_BUILD_DATE "%Y-%m-%dT%H:%M:%SZ" UTC)