Fix Shellcheck SC2006: Use $(...) notation

Use $(...) notation instead of legacy backticked `...`.

https://github.com/koalaman/shellcheck/wiki/SC2006

Signed-off-by: Dan Callahan <danc@element.io>

docker/build_debian.sh

@@ -5,7 +5,7 @@
 set -ex
 
 # Get the codename from distro env
-DIST=`cut -d ':' -f2 <<< "${distro:?}"`
+DIST=$(cut -d ':' -f2 <<< "${distro:?}")
 
 # we get a read-only copy of the source: make a writeable copy
 cp -aT /synapse/source /synapse/build
@@ -17,7 +17,7 @@ cd /synapse/build
 # Section to determine which "component" it should go into (see
 # https://manpages.debian.org/stretch/reprepro/reprepro.1.en.html#GUESSING)
 
-DEB_VERSION=`dpkg-parsechangelog -SVersion`
+DEB_VERSION=$(dpkg-parsechangelog -SVersion)
 case $DEB_VERSION in
     *~rc*|*~a*|*~b*|*~c*)
         sed -ie '/^Section:/c\Section: prerelease' debian/control