Make build_binary_pkgs more verbose

Author: Pavel Marek

mx.fastr/mx_fastr.py

@@ -898,8 +898,12 @@ def build_binary_pkgs(args_in, **kwargs):
         pkgcache(['--print-api-checksum', '--vm', 'fastr'])
     finally:
         sys.stdout = sys.__stdout__
+    checksum_lines = string_io.getvalue().splitlines(keepends=False)
+    mx.log(f"===== Output of 'mx r-pkgcache --print-api-checksum --vm fastr': =====")
+    mx.log(string_io.getvalue())
+    mx.log("===== End of output of 'mx r-pkgcache --print-api-checksum --vm fastr' =====")
     with open(os.path.join(pkgs_path, 'api-checksum.txt'), 'w') as f:
-        checksum = string_io.getvalue().splitlines(keepends=False)[-1]
+        checksum = checksum_lines[-1]
         f.write(checksum)
     # creates the tarball
     result_tarball = os.path.join(dest_dir, pkgs_name + '.tar.gz')