fix: declare the OpenSSL dependency needed on Linux

[MegaRedHand]

What

Declare openssl@3 and pkgconf as Linux-only dependencies of ethrex.

Why

The Linux source build fails in openssl-sys' build script (run 32897389813, ubuntu-24.04):

The system library `openssl` required by crate `openssl-sys` was not found.
The file `openssl.pc` needs to be installed and the PKG_CONFIG_PATH
environment variable must contain its parent directory.
The PKG_CONFIG_PATH environment variable is not set.

ethrex depends on reqwest with its default TLS backend, native-tls:

openssl-sys v0.9.113
├── native-tls v0.2.18
│   └── hyper-tls v0.6.0 → reqwest v0.12.28 → ethrex

On Linux native-tls is OpenSSL, so openssl-sys gets built and probes for the library with pkg-config. Homebrew's superenv builds PKG_CONFIG_PATH from the formula's declared dependencies, so neither the keg-only openssl@3 nor the pkg-config binary was reachable, even though both were already in the tree as transitive dependencies of the build tools.

On macOS native-tls uses Security.framework and openssl-sys is never built, which is why only the Linux job broke. Scoping the dependencies to on_linux leaves the macOS dependency tree, and the existing arm64_sonoma bottle, untouched.

Notes

• brew style and brew audit --skip-style pass locally; brew deps on macOS is unchanged.
• The alternative fix lives upstream in ethrex: switch reqwest to rustls-tls and drop the OpenSSL dependency entirely. Worth doing on its own merits, but it does not help the released 25.0.0 tarball this formula builds.
• The job that hit this was green. ci: stop a formula that fails to build from reporting a green job #21 makes that kind of failure red.