Fix x test rust-installer when cargo is set to a relative path

Previously, this would give an error because the shell script had a
different working directory:

```
test: basic_install
$ sh /home/jyn/src/rust/src/tools/rust-installer/gen-installer.sh --image-dir=/home/jyn/src/rust/src/tools/rust-installer/test/image1 --work-dir=/home/jyn/src/rust/build/x86_64-unknown-linux-gnu/test/rust-installer/workdir --output-dir=/home/jyn/src/rust/build/x86_64-unknown-linux-gnu/test/rust-installer/outdir
/home/jyn/src/rust/src/tools/rust-installer/gen-installer.sh: 15: ../rust3/build/host/stage2-tools-bin/cargo: not found

TEST FAILED!
```

src/bootstrap/config.rs

@@ -1009,7 +1009,9 @@ impl Config {
         });
         config.initial_cargo = build
             .cargo
-            .map(PathBuf::from)
+            .map(|cargo| {
+                t!(PathBuf::from(cargo).canonicalize(), "`initial_cargo` not found on disk")
+            })
             .unwrap_or_else(|| config.out.join(config.build.triple).join("stage0/bin/cargo"));
 
         // NOTE: it's important this comes *after* we set `initial_rustc` just above.