Fix regression of mktree not always rebuilding

Commit d4e808e added cmake caching for
mktree so that we don't rebuild the tree on each "make check"
invocation.  However, this means we can now easily miss some test
dependencies being updated since we don't depend on everything, e.g. the
data/ dir.

To fix this, instead of adding each and every file and directory as a
dependency of the mktree.output command, just rebuild the tree every
time.  The base image (made with DNF) is already cached anyway, and the
added overhead is tiny and outweighs the (potential) engineering hours
lost due to this.

The funny thing is, this was already fixed once in 2019:
fc05045

Sometimes you've just gotta fix stuff twice before it sticks!

Fixes: #2725

tests/CMakeLists.txt

@@ -131,7 +131,10 @@ add_custom_target(reset
 )
 
 add_custom_target(tree
-	DEPENDS mktree.output
+	COMMAND ./mktree build
+	DEPENDS rpmtests
+	DEPENDS ${TESTPROGS}
+	DEPENDS ${TOOLPROGS}
 )
 
 add_custom_command(OUTPUT rpmtests
@@ -140,11 +143,3 @@ add_custom_command(OUTPUT rpmtests
 	DEPENDS ${TESTSUITE_AT}
 	DEPENDS local.at
 )
-
-add_custom_command(OUTPUT mktree.output
-	COMMAND ./mktree build
-	DEPENDS rpmtests
-	DEPENDS ${TESTPROGS}
-	DEPENDS ${TOOLPROGS}
-	DEPENDS mktree
-)

tests/mktree.fedora

@@ -168,9 +168,6 @@ case $CMD in
         # Add RPM installation layer
         rm -rf "$INST_DIR"
         make_install $INST_DIR
-
-        # Signal change to build system
-        touch mktree.output
     ;;
     atshell)
         set -a