fix(ci): use $MCPP variable in fresh user experience steps

ci.yml and ci-macos.yml used bare `mcpp` command which is not in
PATH. ci-windows.yml already used `"$MCPP"` correctly. Align all
three workflows to use the env variable set during bootstrap.

Author: Sunrisepeak

.github/workflows/ci-macos.yml

@@ -323,6 +323,6 @@ jobs:
           # May fail if xlings mcpp version lacks recent fixes.
           TMP=$(mktemp -d)
           cd "$TMP"
-          mcpp new hello
+          "$MCPP" new hello
           cd hello
-          mcpp run
+          "$MCPP" run

.github/workflows/ci.yml

@@ -141,6 +141,6 @@ jobs:
           # May fail if xlings mcpp version lacks recent fixes.
           TMP=$(mktemp -d)
           cd "$TMP"
-          mcpp new hello
+          "$MCPP" new hello
           cd hello
-          mcpp run
+          "$MCPP" run