Add DENO_CERT environment variable for Deno TLS support

[magurotuna]

This adds DENO_CERT environment variable to make Deno programs trust httpjail's proxy CA certificate.

httpjail sets environment variables like SSL_CERT_FILE, NODE_EXTRA_CA_CERTS, etc. to make programs trust the proxy's CA. However, Deno uses its own DENO_CERT variable, causing Deno programs to fail with certificate errors:

root@86de9af35c35:/# httpjail --js "r.host === 'example.com'" -- deno eval "const r = await fetch('https://example.com'); console.log(r.status); await r.bytes();"
03:44:15.504 ERROR httpjail::proxy_tls: TLS handshake failed: received fatal alert: UnknownCA
03:44:15.504 ERROR httpjail::proxy: Error handling HTTPS connection: received fatal alert: UnknownCA
error: Uncaught (in promise) TypeError: error sending request for url (https://example.com/): client error (Connect): invalid peer certificate: UnknownIssuer
const r = await fetch('https://example.com'); console.log(r.status); await r.bytes();
          ^
    at async mainFetch (ext:deno_fetch/26_fetch.js:192:12)
    at async fetch (ext:deno_fetch/26_fetch.js:475:11)
    at async file:///test/$deno$eval.mts:1:11

    hint: Run again with the `--unsafely-ignore-certificate-errors` flag to bypass certificate errors.

To fix this issue, DENO_CERT is added to the list of environment variables set by httpjail. With this patch, Deno throws no errors anymore:

root@905d9a57c5be:/# httpjail --js "r.host === 'example.com'" -- deno eval "const r = await fetch('https://example.com'); console.log(r.status); await r.bytes();"
200

This manual testing was performed using the Dockerfile below.

FROM rust:1.92.0-bookworm AS builder

WORKDIR /build
COPY . .
RUN cargo build --profile fast

FROM debian:bookworm-slim

RUN apt-get update && apt-get install -y \
    curl \
    ca-certificates \
    nftables \
    iproute2 \
    procps \
    unzip \
    && rm -rf /var/lib/apt/lists/*

# Install Node.js 24
RUN curl -fsSL https://deb.nodesource.com/setup_24.x | bash - \
    && apt-get install -y nodejs

# Install Deno
RUN curl -fsSL https://deno.land/install.sh | DENO_INSTALL=/usr/local sh

# Copy httpjail binary
COPY --from=builder /build/target/fast/httpjail /usr/local/bin/httpjail

WORKDIR /
CMD ["/bin/bash"]

Closes #96