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.

src/nix/main.cc

@@ -557,6 +557,8 @@ void mainWrapped(int argc, char * * argv)
 
 int main(int argc, char * * argv)
 {
+    // The CLI has a more detailed version than the libraries; see nixVersion.
+    nix::nixVersion = NIX_CLI_VERSION;
 #ifndef _WIN32
     // Increase the default stack size for the evaluator and for
     // libstdc++'s std::regex.

src/nix/meson.build

@@ -35,6 +35,9 @@ subdir('nix-meson-build-support/windows-version')
 
 configdata = configuration_data()
 
+# The CLI has a more detailed version string than the libraries; see `nixVersion`
+configdata.set_quoted('NIX_CLI_VERSION', meson.project_version())
+
 fs = import('fs')
 
 bindir = get_option('bindir')

src/nix/package.nix

@@ -92,11 +92,11 @@ mkMesonExecutable (finalAttrs: {
   ];
 
   preConfigure =
-    # "Inline" .version so it's not a symlink, and includes the suffix.
+    # Update the repo-global .version file.
-    # Do the meson utils, without modification.
+    # Symlink ./.version points there, but by default only workDir is writable.
     ''
       chmod u+w ./.version
-      echo ${version} > ../../../.version
+      echo ${version} > ./.version
     '';
 
   mesonFlags = [