tests/build-hook.hook.sh: prefer more portable `...` syntax over $(...) for running sub-shells

The /bin/sh interpreter on Solaris doesn't understand $(...) syntax for running
sub-shells. Consequently, this test fails on Solaris. To remedy the situation,
the script either needs to be run by /bin/bash -- which is non-standard --, or
it needs to use the ancient but portable `...` syntax.

tests/build-hook.hook.sh

@@ -6,11 +6,11 @@ drv=$4
 
 echo "HOOK for $drv" >&2
 
-outPath=$(sed 's/Derive(\[("out",\"\([^\"]*\)\".*/\1/' $drv)
+outPath=`sed 's/Derive(\[("out",\"\([^\"]*\)\".*/\1/' $drv`
 
 echo "output path is $outPath" >&2
 
-if $(echo $outPath | grep -q input-1); then
+if `echo $outPath | grep -q input-1`; then
     echo "# accept" >&2
     read x
     echo "got $x"