Pass -msse2 to i686 platforms on CI

In upstream commit 56d3ad9d23bc130aa9404bfdd1957fe81b3ba498,
BoringSSL stopped assuming SSE2 support for i688 platforms,
requiring users to explicitly pass -msse2.

```
  target/i686-unknown-linux-gnu/debug/build/boring-sys-3edff0f2746d7cbb/out/boringssl/crypto/fipsmodule/../internal.h:120:2: error: #error "x86 assembly requires SSE2. Build with -msse2 (recommended), or disable assembly optimizations with -DOPENSSL_NO_ASM."
```

.github/workflows/ci.yml

@@ -108,6 +108,8 @@ jobs:
           rust: stable
           os: ubuntu-latest
           check_only: true
+          custom_env:
+            CXXFLAGS: -msse2
         - thing: x86_64-android
           target: x86_64-linux-android
           rust: stable
@@ -138,6 +140,8 @@ jobs:
           rust: stable
           os: ubuntu-latest
           apt_packages: gcc-multilib g++-multilib
+          custom_env:
+            CXXFLAGS: -msse2
         - thing: arm-linux
           target: arm-unknown-linux-gnueabi
           rust: stable
@@ -184,6 +188,8 @@ jobs:
           target: i686-pc-windows-msvc
           rust: stable-x86_64-msvc
           os: windows-latest
+          custom_env:
+            CXXFLAGS: -msse2
           # CI's Windows doesn't have required root certs
           extra_test_args: --workspace --exclude tokio-boring --exclude hyper-boring
         - thing: x86_64-msvc