Fetch dependencies for -Zbuild-std before entering the sandbox

This allows running `doc -Zbuild-std` from within the sandbox.
Previously, it would error out because cargo tried to download the
standard library's dependencies:

```
[2021-11-27T19:57:24Z INFO  rustwide::cmd] running `Command { std: "docker" "create" "-v" "/home/joshua/src/rust/docs.rs/.workspace/builds/gba-0.5.2/target:/opt/rustwide/target:rw,Z" "-v" "/home/joshua/src/rust/docs.rs/.workspace/builds/gba-0.5.2/source:/opt/rustwide/workdir:ro,Z" "-v" "/home/joshua/src/rust/docs.rs/.workspace/cargo-home:/opt/rustwide/cargo-home:ro,Z" "-v" "/home/joshua/src/rust/docs.rs/.workspace/rustup-home:/opt/rustwide/rustup-home:ro,Z" "-e" "SOURCE_DIR=/opt/rustwide/workdir" "-e" "CARGO_TARGET_DIR=/opt/rustwide/target" "-e" "DOCS_RS=1" "-e" "CARGO_HOME=/opt/rustwide/cargo-home" "-e" "RUSTUP_HOME=/opt/rustwide/rustup-home" "-w" "/opt/rustwide/workdir" "-m" "3221225472" "--user" "1000:1000" "--network" "none" "ghcr.io/rust-lang/crates-build-env/linux-micro" "/opt/rustwide/cargo-home/bin/cargo" "+nightly" "rustdoc" "--lib" "-Zrustdoc-map" "-Z" "unstable-options" "--config" "build.rustdocflags=[\"--cfg\", \"docs_rs\", \"-Z\", \"unstable-options\", \"--emit=invocation-specific\", \"--resource-suffix\", \"-20211126-1.58.0-nightly-6d246f0c8\", \"--static-root-path\", \"/\", \"--cap-lints\", \"warn\", \"--disable-per-crate-search\"]" "-Zunstable-options" "--config=doc.extern-map.registries.crates-io=\"https://docs.rs/{pkg_name}/{version}/thumbv4t-none-eabi\"" "-Zbuild-std" "--target" "thumbv4t-none-eabi", kill_on_drop: false }`
[2021-11-27T19:57:24Z INFO  rustwide::cmd] [stdout] fe2773f8a17ab13ce7b66c7221f91883a7b92b3bdeef7b30bf2ed55aa6b3d511
[2021-11-27T19:57:24Z INFO  rustwide::cmd] running `Command { std: "docker" "start" "-a" "fe2773f8a17ab13ce7b66c7221f91883a7b92b3bdeef7b30bf2ed55aa6b3d511", kill_on_drop: false }`
[2021-11-27T19:57:24Z INFO  rustwide::cmd] [stderr]  Downloading crates ...
[2021-11-27T19:57:24Z INFO  rustwide::cmd] [stderr] warning: spurious network error (2 tries remaining): [6] Couldn't resolve host name (Could not resolve host: crates.io)
[2021-11-27T19:57:24Z INFO  rustwide::cmd] [stderr] error: failed to download from `https://crates.io/api/v1/crates/libc/0.2.106/download`
[2021-11-27T19:57:24Z INFO  rustwide::cmd] [stderr]
[2021-11-27T19:57:24Z INFO  rustwide::cmd] [stderr] Caused by:
[2021-11-27T19:57:24Z INFO  rustwide::cmd] [stderr]   [6] Couldn't resolve host name (Could not resolve host: crates.io)
```

src/prepare.rs

@@ -140,7 +140,7 @@ impl<'a> Prepare<'a> {
     fn fetch_deps(&mut self) -> Result<(), Error> {
         let mut missing_deps = false;
         let res = Command::new(self.workspace, self.toolchain.cargo())
-            .args(&["fetch", "--manifest-path", "Cargo.toml"])
+            .args(&["fetch", "-Zbuild-std", "--manifest-path", "Cargo.toml"])
             .cd(&self.source_dir)
             .process_lines(&mut |line, _| {
                 if line.contains("failed to load source for dependency") {