Give the Linux server 64 bits of session entropy and a pinned Steam lib

Two follow-ups to the Linux server support in #6. Both are Linux-only, so
neither reached a Windows player.

Session codes
- nonce()'s fallback for a container with no /dev/urandom read
  "i < 4 && !value", which ended the loop as soon as the first round
  produced anything: one round of sixteen bits instead of four. Over
  200k draws that is 62k distinct codes with a ceiling of 0xffff, rather
  than 200k across the full range.

libsteam_api.so
- It was fetched from a mirror's main branch with no hash, by CI and by
  the script every server operator runs, and it is loaded into the
  server process. A branch name is whatever that branch holds on the day
  it runs.
- Pinned to mirror commit df2baabf574a and checked against its SHA-256
  (385840 bytes, eb2dd015...). The size and export checks already there
  catch a wrong file; a hash is what catches a swapped one.
- --sdk-url still overrides the URL and drops the check, since a
  different URL has a different hash; --sdk-sha256 keeps it.

Server/server_host.cpp

@@ -32,7 +32,9 @@ std::uint64_t nonce() {
     std::ifstream urandom("/dev/urandom", std::ios::binary);
     if (urandom.read(reinterpret_cast<char *>(&value), sizeof(value)) && value) return value;
     std::random_device device;
-    for (int i = 0; i < 4 && !value; ++i)
+    // Four rounds of sixteen bits each. Stopping as soon as `value` was
+    // non-zero would leave a session code with sixteen bits of entropy.
+    for (int i = 0; i < 4; ++i)
         value = (value << 16) ^ static_cast<std::uint64_t>(device() & 0xFFFF);
     if (!value) throw std::runtime_error("Cannot generate a session code.");
 #endif