build.rs: make wasm-ast root configurable with an env-var

[marijanp]

In a sandboxed build environment, cargo metadata tries to query crates.io. Even though I have been trying to pass --offline or frozen, I ran into an exception:

golem>   thread 'main' panicked at /build/cargo-vendor-dir/golem-wasm-rpc-1.0.2/build.rs:29:10:
golem>   called `Result::unwrap()` on an `Err` value: CargoMetadata { stderr: "warning: `/build/cargo-vendor-dir/.cargo/config` is deprecated in favor of `config.toml`
note: if you need to support cargo 1.38 or earlier, you can symlink `config` to `config.toml`
warning: `/build/.cargo/config` is deprecated in favor of `config.toml`
note: if you need to support cargo 1.38 or earlier, you can symlink `config` to `config.toml`
error: failed to get `arbitrary` as a dependency of package `golem-wasm-rpc v1.0.2 (/build/cargo-vendor-dir/golem-wasm-rpc-1.0.2)`

Caused by:
  failed to load source for dependency `arbitrary`
Caused by:
  Unable to update registry `crates-io`
Caused by:
  failed to update replaced source registry `crates-io`
Caused by:
  failed to read root of directory source: /build/cargo-vendor-dir/cargo-vendor-dir

Caused by:
  No such file or directory (os error 2)
" }

I want to propose a way to inject the location of the crate by setting an environment variable. If the environment variable is not set, we default to use cargo metadata

An analogous solution was implemented for golem-examples: golemcloud/golem-examples#65