Fix linux installation for newer Unity versions (#387)

Description
============

It seems a very old bug suddenly surfaced.
The linux pkg install logic is reading the contents of the archive
into memory and passing it down to the stdin for the `cpio` command.
I used the singe `write` call to copy the `buffer` to the `stdin` without
checking if all bytes are actually written. It seems pure luck that this
worked until now. It seems that in recent versions of Unity some packages
got so big that a single call to `write` wasn't writing all the bytes into the
memory location.

I replaced the logic with a filestream copy instead. Something I should have done
to begin with ...

install/uvm_install_core/src/sys/linux/pkg.rs

@@ -2,7 +2,7 @@ use crate::*;
 use std::ffi::OsStr;
 use std::fs::DirBuilder;
 use std::io::Read;
-use std::io::Write;
+use std::io::{self, BufRead, BufReader, Write};
 use std::path::Path;
 use std::process::{Command, Stdio};
 
@@ -61,11 +61,10 @@ impl ModulePkgInstaller {
                 .stdin(Stdio::piped())
                 .spawn()?;
             {
-                let stdin = cpio.stdin.as_mut().expect("stdin");
+                let stdin = cpio.stdin.as_mut().ok_or("Failed to open cpio stdin")?;
                 let mut file = File::open(payload)?;
-                let mut buffer = Vec::new();
-                file.read_to_end(&mut buffer)?;
-                stdin.write(&buffer)?;
+                let mut reader = BufReader::new(file);
+                io::copy(&mut reader, stdin)?;
             }
             cpio
         } else {
@@ -81,15 +80,14 @@ impl ModulePkgInstaller {
                 .stdin(Stdio::piped())
                 .spawn()?;
             {
-                let stdin = cpio.stdin.as_mut().expect("stdin");
-                let gzip_std_out = gzip.stdout.as_mut().expect("stdout");
-                let mut buffer = Vec::new();
-                gzip_std_out.read_to_end(&mut buffer)?;
-                stdin.write(&buffer)?;
+                let gzip_stdout = gzip.stdout.as_mut().ok_or("Failed to open gzip stdout")?;
+                let cpio_stdin = cpio.stdin.as_mut().ok_or("Failed to open cpio stdin")?;
+
+                io::copy(gzip_stdout, cpio_stdin)?;
             }
             cpio
         };
-
+    
         let tar_output = tar_child.wait_with_output()?;
         if !tar_output.status.success() {
             return Err(format!(