shellcheck: Fix SC2006 issue

nx-X11/x-indent-all.sh

 #!/bin/sh
-where=`dirname $0`
+where=$(dirname $0)
 git ls-files | grep '\.[chm]$' | xargs sh "$where"/x-indent.sh
 

nx-X11/x-indent.sh

@@ -2,7 +2,7 @@
 
 # We want GNU indent, so first search for gindent to avoid /usr/bin/indent
 # on the BSDs, which won't work for us
-INDENT=`which gnuindent || which gindent || which indent`
+INDENT=$(which gnuindent || which gindent || which indent)
 
 if [ -z "${INDENT}" ] ; then
     echo "Could not find indent, sorry..." >&2

roll-tarballs.sh

@@ -59,7 +59,7 @@ fi
 if ! git rev-parse --verify -q "$CHECKOUT" >/dev/null; then
     echo "   '${RELEASE}' is not a valid release number because there is no git tag named ${CHECKOUT}."
     echo "   Please specify one of the following releases:"
-    echo "HEAD (on branch `git rev-parse --abbrev-ref HEAD`)"
+    echo "HEAD (on branch $(git rev-parse --abbrev-ref HEAD))"
     git tag -l | grep "^redist" | cut -f2 -d"/" | sort -u
     exit 1
 fi