chore(release): bootstrap pin -> 0.0.61 + fix install.sh NO_PATH unbound var

- .xlings.json: bump self-host bootstrap pin to 0.0.61, now that it's
  released + mirrored (xlings-res/mcpp gh+gitcode) + indexed (xim-pkgindex
  mcpp.lua latest=0.0.61, artifact republished). CI now bootstraps from 0.0.61.
- install.sh: the final 'source $rc' hint referenced $rc, which is only set
  in the PATH-integration branch — with MCPP_NO_PATH set, 'set -u' tripped an
  unbound-variable error AFTER a successful install (non-zero exit). Guard the
  message: print the source hint only when rc is set, else a PATH hint.

Author: Sunrisepeak

.xlings.json

@@ -1,5 +1,5 @@
 {
   "workspace": {
-    "mcpp": "0.0.60"
+    "mcpp": "0.0.61"
   }
 }

install.sh

@@ -158,4 +158,8 @@ echo
 "$PREFIX/bin/mcpp" --version
 echo
 echo "✓ mcpp installed at $PREFIX"
-echo "  Open a new shell (or 'source $rc') and run:  mcpp --help"
+if [[ -n "${rc:-}" ]]; then
+    echo "  Open a new shell (or 'source $rc') and run:  mcpp --help"
+else
+    echo "  Add $PREFIX/bin to your PATH, then run:  mcpp --help"
+fi