fix(build): make build_protos.sh POSIX sh compatible

build_protos.sh used bash arithmetic syntax (( … )) while being invoked via /bin/sh, causing runtime failures on systems where /bin/sh is dash.

Use POSIX-compliant [ … ] numeric tests instead.

This fixes runtime errors like:

$ ./build_protos.sh ./build_protos.sh: 24: REGEN_RESULT: not found ./build_protos.sh: 24: REGEN_CONFIG: not found ./build_protos.sh: 33: REGEN_RESULT: not found ./build_protos.sh: 40: REGEN_CONFIG: not found