Honor the rustfmt setting in config.toml

Prior to this, setting the rustfmt configuration was ignored:

```
% mkdir example

% cd example

% ../configure --set build.rustfmt=/usr/bin/true

% ../x.py fmt
./x.py fmt is not supported on this channel
failed to run: /Users/shep/Projects/rust/example/build/bootstrap/debug/bootstrap fmt
Build completed unsuccessfully in 0:00:01
```

And after:

```
% ../x.py fmt
Build completed successfully in 0:00:11
```

1 files changed, 12 insertions, 5 deletions

diff --git a/src/bootstrap/config.rs b/src/bootstrap/config.rs
index 7698ff62880..40ea9afb6f6 100644
--- a/src/bootstrap/config.rs
+++ b/src/bootstrap/config.rs
@@ -913,11 +913,18 @@ impl Config {
             set(&mut config.missing_tools, t.missing_tools);
         }
 
-        // Cargo does not provide a RUSTFMT environment variable, so we
-        // synthesize it manually. Note that we also later check the config.toml
-        // and set this to that path if necessary.
-        let rustfmt = config.initial_rustc.with_file_name(exe("rustfmt", config.build));
-        config.initial_rustfmt = if rustfmt.exists() { Some(rustfmt) } else { None };
+        config.initial_rustfmt = config.initial_rustfmt.or_else({
+            let build = config.build;
+            let initial_rustc = &config.initial_rustc;
+
+            move || {
+                // Cargo does not provide a RUSTFMT environment variable, so we
+                // synthesize it manually.
+                let rustfmt = initial_rustc.with_file_name(exe("rustfmt", build));
+
+                if rustfmt.exists() { Some(rustfmt) } else { None }
+            }
+        });
 
         // Now that we've reached the end of our configuration, infer the
         // default values for all options that we haven't otherwise stored yet.