chore: pin to hex=0.4.2, build without std in CI check

hex=0.4.3 was breaking the build, see:
KokaKiwi/rust-hex#55

cargo-nono was reporting false positives on zeroize and
num-bigint, so instead of using that in CI I'm instead
building without the default `std` feature

.github/workflows/rust.yml

@@ -52,17 +52,21 @@ jobs:
           args: --all -- --check
 
   no-std:
-   name: no-std compatibility check
-   runs-on: ubuntu-latest
-   steps:
-     - uses: actions/checkout@v2
-     - uses: actions-rs/toolchain@v1
-       with:
-         profile: minimal
-         toolchain: stable
-         override: true
-     - run: cargo install --locked cargo-nono
-     - run: cargo nono check --no-default-features
+    name: no_std compatibility check
+      runs-on: ubuntu-latest
+      steps:
+        - uses: actions/checkout@v2
+        - uses: actions-rs/toolchain@v1
+          with:
+            profile: minimal
+            toolchain: stable
+            override: true
+        - uses: Swatinem/rust-cache@v1
+        - uses: actions-rs/cargo@v1
+        - name: Build with --no-default-features
+              run: |
+                cargo check --no-default-features
+                cargo build --no-default-features
 
   # clippy:
   #  name: Clippy

Cargo.toml

@@ -9,8 +9,8 @@ license = "MIT OR Apache-2.0"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
-hex = {version ="0.4", default-features=false}
-num-bigint = {version= "0.4", default-features=false}
+hex = {version ="=0.4.2", default-features=false}
+num-bigint = {version= "0.4.4", default-features=false}
 once_cell = {version= "1.8", default-features=false}
 tracing = {version = "0.1", default-features=false}
 tracing-subscriber = {version = "0.3", default-features=false }
@@ -25,7 +25,7 @@ ark-bls12-377 = "0.4"
 ark-ed-on-bls12-377 = { version = "0.4", features = ["r1cs"] }
 ark-groth16 = { version = "0.4", default-features=false, optional=true }
 ark-snark = { version = "0.4", optional=true }
-zeroize = {version ="1.4", default-features=false}
+zeroize = {version ="1.7", default-features=false}
 hashbrown = "0.14.3"
 
 # This matches what ark-std (a library for no_std compatibility) does, having