clippy: warn disallowed_methods for std::env::var and friends

In 11588 we want to avoid reading from environment variables,
11727 did that well. I wonder if we could leverage tools to help
with this. Thankfully, clippy has a `disallowed_methods`[1] lint,
helping us enforce the rule.

[1]: https://rust-lang.github.io/rust-clippy/stable/index.html#disallowed_methods

clippy.toml

@@ -0,0 +1,6 @@
+disallowed-methods = [
+    { path = "std::env::var", reason = "Use `Config::get_env` instead. See rust-lang/cargo#11588" },
+    { path = "std::env::var_os", reason = "Use `Config::get_env_os` instead. See rust-lang/cargo#11588" },
+    { path = "std::env::vars", reason = "Not recommended to use in Cargo. See rust-lang/cargo#11588" },
+    { path = "std::env::vars_os", reason = "Not recommended to use in Cargo. See rust-lang/cargo#11588" },
+]

src/bin/cargo/main.rs

@@ -1,5 +1,6 @@
 #![warn(rust_2018_idioms)] // while we're getting used to 2018
 #![allow(clippy::all)]
+#![warn(clippy::disallowed_methods)]
 
 use cargo::util::toml::StringOrVec;
 use cargo::util::CliError;

src/cargo/lib.rs

@@ -4,6 +4,7 @@
 // Due to some of the default clippy lints being somewhat subjective and not
 // necessarily an improvement, we prefer to not use them at this time.
 #![allow(clippy::all)]
+#![warn(clippy::disallowed_methods)]
 #![warn(clippy::self_named_module_files)]
 #![allow(rustdoc::private_intra_doc_links)]