Export the same DNF command-line in "make env"

When managing the test tree with DNF, it's actually desirable to use the
same options that were used during the tree construction (especially the
cachedir), so just do that.

tests/mktree.fedora

@@ -54,6 +54,18 @@ simply like this:
 
 CMD=$1; shift
 
+dnf()
+{
+    HOME=$CACHE_DIR command dnf \
+        --installroot=$BASE_DIR \
+        --releasever=@OS_VERSION@ \
+        --setopt=cachedir=$CACHE_DIR/dnf \
+        --setopt=keepcache=1 \
+        --setopt=install_weak_deps=0 \
+        --disablerepo=\* --enablerepo=fedora,updates \
+        --exclude=rpm,systemd-udev "$@"
+}
+
 mount_tree()
 {
     source ./atlocal
@@ -85,15 +97,7 @@ case $CMD in
         mkdir -p $CACHE_DIR
         echo "$RPM_MACROS" > $CACHE_DIR/.rpmmacros
 
-        HOME=$CACHE_DIR \
         dnf install -y \
-            --installroot=$BASE_DIR \
-            --releasever=@OS_VERSION@ \
-            --setopt=cachedir=$CACHE_DIR/dnf \
-            --setopt=keepcache=1 \
-            --setopt=install_weak_deps=0 \
-            --disablerepo=\* --enablerepo=fedora,updates \
-            --exclude=rpm,systemd-udev \
             bash \
             binutils \
             bubblewrap \
@@ -151,7 +155,11 @@ case $CMD in
     env)
         set -a
         mount_tree $SANDBOX_DIR
-        dnf() { command dnf --installroot=$RPMTEST "$@"; }
+
+        export CACHE_DIR
+        export BASE_DIR=$RPMTEST
+        export -f dnf
+
         echo "$ENV_MOTD"
         $SHELL
         exit 0