Restore detailed Nix CLI version

... as intended.

Requirements:
- don't build fresh libraries for each git commit
- have git commit in the CLI

Bug:
- echo ${version} went into the wrong file => use the fact that it's
  a symlink, not just for reading but also for writing.

src/libstore/globals.cc

@@ -242,7 +242,7 @@ Path Settings::getDefaultSSLCertFile()
     return "";
 }
 
-const std::string nixVersion = PACKAGE_VERSION;
+std::string nixVersion = PACKAGE_VERSION;
 
 NLOHMANN_JSON_SERIALIZE_ENUM(SandboxMode, {
     {SandboxMode::smEnabled, true},

src/libstore/globals.hh

@@ -1248,7 +1248,15 @@ void loadConfFile(AbstractConfig & config);
 // Used by the Settings constructor
 std::vector<Path> getUserConfigFiles();
 
-extern const std::string nixVersion;
+/**
+ * The version of Nix itself.
+ *
+ * This is not `const`, so that the Nix CLI can provide a more detailed version
+ * number including the git revision, without having to "re-compile" the entire
+ * set of Nix libraries to include that version, even when those libraries are
+ * not affected by the change.
+ */
+extern std::string nixVersion;
 
 /**
  * @param loadConfig Whether to load configuration from `nix.conf`, `NIX_CONFIG`, etc. May be disabled for unit tests.