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).
Frank Hunleth
cff8010e80