net5.0/net6.0/netstandard builds pick a non-executable file earlier on the overlay PATH, so the command fails with "Permission denied" instead of running the executable copy later on #139

What's wrong

TryExecutable (RunCommand/RunCommand.cs, around line 510) skips files with no execute bit. That check sits inside #if NET7_0_OR_GREATER, because it uses File.GetUnixFileMode. The net5.0, net6.0, netstandard2.0 and netstandard2.1 builds therefore accept any file that exists.

On Linux and macOS those builds can pick a non-executable file over a real executable later on the overlay PATH. Neither the README ("On other platforms the file must have an execute bit") nor the method's XML doc mentions this limit.

ExecuteAsyncShouldSkipANonExecutableFileOnTheOverlayPath passes only because the test project targets net10.0, so it never exercises those builds. The behaviour came in with #137.

Repro (Linux)

The repro is a net6.0 console app that references the library's net6.0 build. The #if is decided at compile time, so the runtime version doesn't matter.

/tmp/rc-noexec/rctool   mode 644, prints "non-executable copy"
/tmp/rc-exec/rctool     mode 755, prints "executable copy"

var opts = new CommandOptions { EnvironmentVariables = new Dictionary<string, string?> { ["PATH"] = "/tmp/rc-noexec:/tmp/rc-exec:/usr/bin:/bin" } };
await RunCommand.ExecuteAsync("rctool", [], new LineOutputHandler(Console.WriteLine, Console.WriteLine), opts, CancellationToken.None);

• Observed: Win32Exception: An error occurred trying to start process '/tmp/rc-noexec/rctool' ... Permission denied
• Expected: executable copy, which is what PATH=/tmp/rc-noexec:/tmp/rc-exec sh -c rctool prints.

Suggested fix

• Builds below net7: on non-Windows platforms, check with libc access(path, X_OK) through P/Invoke. Also consider doing this on net7+. The current "any execute bit" test accepts a 0700 file owned by another user, which the caller can't run, so a shell would skip it and RunCommand doesn't.
• Tests: run the overlay-PATH tests against a pre-net7 target too, or add a unit test of TryExecutable that is compiled for those TFMs.