Fix System.cmd/3 unit test that fails on Ubuntu 26.04

[fhunleth]

This test runs System.cmd/3 against a symlink to the echo command. This works when echo is its own binary and when it's in a multicall binary that's dispatched using argv[0]. It doesn't work for the new Rust/uutils coreutils which doesn't dispatch via argv[0]. In this case, intended call to echo produces no output:

  1) test Unix cmd/3 with absolute and relative paths (SystemTest)
     test/elixir/system_test.exs:210
     match (=) failed
     code:  assert {"hello\n", 0} = System.cmd(Path.join(File.cwd!(), @echo), ["hello"], arg0: "echo")
     left:  {"hello\n", 0}
     right: {"", 0}
     stacktrace:
       test/elixir/system_test.exs:223: anonymous fn/0 in SystemTest."test Unix cmd/3 with absolute and relative paths"/1
       (elixir 1.21.0-dev) lib/file.ex:2100: File.cd!/2
       test/elixir/system_test.exs:215: (test)

This fix switches the binary used for the test over to sh which fixes the issue on Ubuntu 26.04. This was also verified on Ubuntu 24.04, Ubuntu 22.04, Alpine Linux 3.24, macOS Tahoe (26.5.1) and MacOS Sequoia (15.3.2).

[josevalim]

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