script: Enable SC2006 rule for Gitian scripts

test/lint/lint-shell.sh

@@ -24,7 +24,6 @@ disabled=(
 )
 disabled_gitian=(
     SC2001 # See if you can use ${variable//search/replace} instead.
-    SC2006 # Use $(...) notation instead of legacy backticked `...`.
     SC2094 # Make sure not to read and write the same file in the same pipeline.
     SC2129 # Consider using { cmd1; cmd2; } >> file instead of individual redirects.
     SC2155 # Declare and assign separately to avoid masking return values.