pgo: explicitly set LLVM_PROFILE_FILE envvar

When building foot with pgo under gentoo's portage, LLVM tried to
generate profile data files directly under system root, triggering
sandbox violation and causing build to fail. Setting this envvar fixes
the issue by explicitly specifying profiling data location.

Reference: https://clang.llvm.org/docs/UsersManual.html#profiling-with-instrumentation

pgo/pgo.sh

@@ -103,6 +103,7 @@ if [ ${do_pgo} = yes ]; then
     ninja -C "${blddir}" test
 
     # Run mode-dependent script to generate profiling data
+    export LLVM_PROFILE_FILE="${blddir}/default_%m.profraw"
     "${srcdir}"/pgo/${mode}.sh "${srcdir}" "${blddir}"
 
     if [ ${compiler} = clang ]; then