Fix infinite recursion in bash completions when rustc is not in PATH

Previously, running `source /etc/bash_completion.d/cargo` would loop
forever:

```
Command 'rustc' not found, but can be installed with:

sudo snap install rustup  # version 1.24.3, or
sudo apt  install rustc   # version 1.47.0+dfsg1+llvm-1ubuntu1~20.04.1

See 'snap info rustup' for additional versions.

Command 'rustc' not found, but can be installed with:

sudo snap install rustup  # version 1.24.3, or
sudo apt  install rustc   # version 1.47.0+dfsg1+llvm-1ubuntu1~20.04.1

See 'snap info rustup' for additional versions.

Command 'rustc' not found, but can be installed with:
```

The issue was that `$(rustc --print sysroot)/etc/bash_completion.d/cargo`
will expand to `/etc/bash_completion.d/cargo` is rustc doesn't print
anything, which sources the same file over and over. This fixes it by
first checking that rustc exists.

This also adds quotes around the sysroot, to support spaces with
filenames.

Author: jyn514

src/cli/rustup_mode.rs

@@ -1686,7 +1686,9 @@ fn output_completion_script(shell: Shell, command: CompletionCommand) -> Result<
 
             writeln!(
                 &mut term2::stdout(),
-                "source $(rustc --print sysroot){}",
+                "if command -v rustc >/dev/null 2>&1; then\n\
+                    \tsource \"$(rustc --print sysroot)\"{}\n\
+                 fi",
                 script,
             )?;
         }