ci: build test helpers before the valgrind run

`make` alone does not build the CHECK_PROGS test helpers (tls, trimslash,
t_chmod_secure, ...), so runtests.py exited immediately with "missing
test helper program(s)", produced no valgrind logs, and the scan step
failed every job with "the suite did not run". Use `make check-progs`,
which builds rsync plus the helpers and symlink fixtures without running
the suite.

.github/workflows/valgrind.yml

@@ -38,7 +38,7 @@ jobs:
     - name: configure
       run: ./configure --with-rrsync --enable-debug
     - name: make
-      run: make
+      run: make check-progs   # builds rsync + the test helper programs runtests.py needs
     - name: info
       run: ./rsync --version