Bump minimum Rust version to 1.33

[ebkalderon]

It seems that percent-encoding 2.1.0 is using let bindings in const fn which is causing this CI build to fail on stable Rust 1.31 with the following error:

error: local variables in const fn are unstable
  --> /usr/local/cargo/registry/src/github.com-1ecc6299db9ec823/percent-encoding-2.1.0/lib.rs:72:13
   |
72 |         let chunk = self.mask[byte as usize / BITS_PER_CHUNK];
   |             ^^^^^

This feature was only stabilized in Rust 1.33 according to this comment. As such, we should probably bump ebkalderon/renderdoc-rs-circleci-dockerfile to a minimum Rust compiler version 1.33.

This issue is a counterpart to ebkalderon/renderdoc-rs#50.