cargo deny: ignore RUSTSEC-2021-0064

Transient dependencies depend on two different versions of `cpuid-bool`.

This advisory does not appear to be urgent. We should review this ignore
after a few weeks to see if our deps have switched over.

text of the advisory:

Issued
May 6, 2021
Package
cpuid-bool (crates.io)
Type
Unmaintained
Details
https://github.com/RustCrypto/utils/pull/381
Patched
no patched versions
Description
Please use the `cpufeatures`` crate going forward:

https://github.com/RustCrypto/utils/tree/master/cpufeatures

There will be no further releases of cpuid-bool.

deny.toml

@@ -14,6 +14,7 @@ vulnerability = "deny"
 unmaintained = "deny"
 yanked = "deny"
 ignore = [
+    "RUSTSEC-2021-0064"
 ]
 
 # https://embarkstudios.github.io/cargo-deny/checks/licenses/cfg.html