mkosi.build: print important build commands

This undoes the effect of 1394a3ec35 partially.
We print the fairly verbose output of the build commands, so let's also
print the commands themselves. This makes it much easier to understand what
is going on.

(The style was copied from other scripts where we do 'set -x' for one command.)

mkosi.presets/00-base/mkosi.build

@@ -174,10 +174,10 @@ if [ ! -f "$BUILDDIR"/build.ninja ]; then
         )
     fi
 
-    meson setup "$BUILDDIR" "${CONFIGURE_OPTS[@]}"
+    ( set -x; meson setup "$BUILDDIR" "${CONFIGURE_OPTS[@]}" )
 fi
 
-ninja -C "$BUILDDIR" "$@"
+( set -x; ninja -C "$BUILDDIR" "$@" )
 if [ "$WITH_TESTS" = 1 ]; then
     if [ -n "$SANITIZERS" ]; then
         export ASAN_OPTIONS="$MKOSI_ASAN_OPTIONS"
@@ -187,7 +187,7 @@ if [ "$WITH_TESTS" = 1 ]; then
         TIMEOUT_MULTIPLIER=1
     fi
 
-    meson test -C "$BUILDDIR" --print-errorlogs --timeout-multiplier=$TIMEOUT_MULTIPLIER
+    ( set -x; meson test -C "$BUILDDIR" --print-errorlogs --timeout-multiplier=$TIMEOUT_MULTIPLIER )
 fi
 
-meson install -C "$BUILDDIR" --quiet --no-rebuild --only-changed
+( set -x; meson install -C "$BUILDDIR" --quiet --no-rebuild --only-changed )