[Optional] Improve the build check of dcap-ql

[Taowyoo]

The default std I added for mbedtls in dcap-ql is for avoiding the compiling error when compile the dcap-ql with only verify and with mbedtls specify with default-features = false in a crate e.g.:

[package]
name = "dcap-ql-fail"
version = "0.1.0"
edition = "2021"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[dependencies]
dcap-ql = { version = "0.3.6", default-features = false, features = ["verify"] }
mbedtls = { version = "0.9.0", default-features = false}

Because mbedtls needs at least one of std or no_std_deps features is defined.

Reason
After some research, the problem actually is from cargo. cargo will automatically include dev dependencies when running cargo build. Ref: rust-lang/cargo#4988

So cargo build -p dcap-ql --no-default-features --features verify will succeed since there is a dev dependency of mbedtls with default feature.

And if use cargo build -p dcap-ql --no-default-features --features verify -Z avoid-dev-deps to exclude dev dependencies , then I found some other dependencies also has this problem which cause compilation failed.

And seems current only way is:

For people who are using stable version numbers to cargo check MSRV (so can't use nightly), there is a dirty hacky workaround.

Originally posted by @Taowyoo in #498 (comment)