Fix isatty in WASI. (#3696)

WASI doesn't have an `isatty` ioctl or syscall, so wasi-libc's `isatty`
implementation uses the file descriptor type and rights to determine if
the file descriptor is likely to be a tty. The real fix here will be to
add an `isatty` call to WASI. But for now, have Wasmtime set the
filetype and rights for file descriptors so that wasi-libc's `isatty`
works as expected.

crates/wasi-common/src/pipe.rs

@@ -180,6 +180,9 @@ impl<R: Read + Any + Send + Sync> WasiFile for ReadPipe<R> {
     async fn num_ready_bytes(&self) -> Result<u64, Error> {
         Ok(0)
     }
+    fn isatty(&self) -> bool {
+        false
+    }
     async fn readable(&self) -> Result<(), Error> {
         Err(Error::badf())
     }
@@ -336,6 +339,9 @@ impl<W: Write + Any + Send + Sync> WasiFile for WritePipe<W> {
     async fn num_ready_bytes(&self) -> Result<u64, Error> {
         Ok(0)
     }
+    fn isatty(&self) -> bool {
+        false
+    }
     async fn readable(&self) -> Result<(), Error> {
         Err(Error::badf())
     }