gnu: ninja: Patch hard-coded /bin/echo reference.

Solves a test failure on systems where open file limits are high enough for
this test to run.  See <https://bugs.gnu.org/30601>.

* gnu/packages/ninja.scm (ninja)[arguments]: Add substitution.

gnu/packages/ninja.scm

@@ -47,6 +47,7 @@
          (replace 'configure
            (lambda _
              (substitute* "src/subprocess-posix.cc"
+               (("/bin/echo") (which "echo"))
                (("/bin/sh") (which "sh")))
              #t))
          (replace 'build