Add a setting for configuring the SSL certificates file

This provides a platform-independent way to configure the SSL
certificates file in the Nix daemon. Previously we provided
instructions for overriding the environment variable in launchd, but
that obviously doesn't work with systemd. Now we can just tell users
to add

  ssl-cert-file = /etc/ssl/my-certificate-bundle.crt

to their nix.conf.

src/libstore/filetransfer.cc

@@ -318,7 +318,7 @@ struct curlFileTransfer : public FileTransfer
 
             if (request.verifyTLS) {
                 if (settings.caFile != "")
-                    curl_easy_setopt(req, CURLOPT_CAINFO, settings.caFile.c_str());
+                    curl_easy_setopt(req, CURLOPT_CAINFO, settings.caFile.get().c_str());
             } else {
                 curl_easy_setopt(req, CURLOPT_SSL_VERIFYPEER, 0);
                 curl_easy_setopt(req, CURLOPT_SSL_VERIFYHOST, 0);

src/libstore/globals.cc

@@ -44,14 +44,9 @@ Settings::Settings()
     lockCPU = getEnv("NIX_AFFINITY_HACK") == "1";
     allowSymlinkedStore = getEnv("NIX_IGNORE_SYMLINK_STORE") == "1";
 
-    caFile = getEnv("NIX_SSL_CERT_FILE").value_or(getEnv("SSL_CERT_FILE").value_or(""));
-    if (caFile == "") {
-        for (auto & fn : {"/etc/ssl/certs/ca-certificates.crt", "/nix/var/nix/profiles/default/etc/ssl/certs/ca-bundle.crt"})
-            if (pathExists(fn)) {
-                caFile = fn;
-                break;
-            }
-    }
+    auto sslOverride = getEnv("NIX_SSL_CERT_FILE").value_or(getEnv("SSL_CERT_FILE").value_or(""));
+    if (sslOverride != "")
+        caFile = sslOverride;
 
     /* Backwards compatibility. */
     auto s = getEnv("NIX_REMOTE_SYSTEMS");
@@ -187,6 +182,13 @@ bool Settings::isWSL1()
     return hasSuffix(utsbuf.release, "-Microsoft");
 }
 
+Path Settings::getDefaultSSLCertFile()
+{
+    for (auto & fn : {"/etc/ssl/certs/ca-certificates.crt", "/nix/var/nix/profiles/default/etc/ssl/certs/ca-bundle.crt"})
+        if (pathExists(fn)) return fn;
+    return "";
+}
+
 const std::string nixVersion = PACKAGE_VERSION;
 
 NLOHMANN_JSON_SERIALIZE_ENUM(SandboxMode, {

src/libstore/globals.hh

@@ -64,6 +64,8 @@ class Settings : public Config {
 
     bool isWSL1();
 
+    Path getDefaultSSLCertFile();
+
 public:
 
     Settings();
@@ -826,8 +828,17 @@ public:
           > `.netrc`.
         )"};
 
-    /* Path to the SSL CA file used */
-    Path caFile;
+    Setting<Path> caFile{
+        this, getDefaultSSLCertFile(), "ssl-cert-file",
+        R"(
+          The path of a file containing CA certificates used to
+          authenticate `https://` downloads. It defaults to the first
+          of `/etc/ssl/certs/ca-certificates.crt` and
+          `/nix/var/nix/profiles/default/etc/ssl/certs/ca-bundle.crt`
+          that exists. It can be overriden using the
+          `NIX_SSL_CERT_FILE` and `SSL_CERT_FILE` environment variable
+          (in that order of precedence).
+        )"};
 
 #if __linux__
     Setting<bool> filterSyscalls{