mux: allow build-time version label override (FREEMKV_BUILD_LABEL)

build.rs emits FREEMKV_VERSION = FREEMKV_BUILD_LABEL when that env is set
(non-empty), else the Cargo package version. The muxing/writing-application
field and the FVI generator tag use it, so a pre-release/test build can be
stamped without bumping Cargo.toml and disturbing the tag-pinned [patch]
version matching. The git short hash is still appended either way.

src/mux/mkv.rs

@@ -774,7 +774,7 @@ impl<W: Write + Seek> MkvMuxer<W> {
         // Stamp the freemkv version so any muxed file is traceable to the build
         // that produced it (MediaInfo "Writing application"/"library").
         const FREEMKV_MUX_APP: &str =
-            concat!("freemkv ", env!("CARGO_PKG_VERSION"), env!("GIT_SUFFIX"));
+            concat!("freemkv ", env!("FREEMKV_VERSION"), env!("GIT_SUFFIX"));
         ebml::write_string(&mut writer, ebml::MUXING_APP, FREEMKV_MUX_APP)?;
         ebml::write_string(&mut writer, ebml::WRITING_APP, FREEMKV_MUX_APP)?;
         if let Some(t) = title {