build: track the workspace cargo config (FFMPEG_DIR, OCIO env)

FFMPEG_DIR is relative and portable; OCIO_INSTALL_DIR=/usr is this
machine's system-OCIO preference (CI overrides it via GITHUB_ENV;
delete that line to use the vendored static OCIO build elsewhere).

.cargo/config.toml

@@ -0,0 +1,4 @@
+[env]
+FFMPEG_DIR = { value = ".cache/ffmpeg", relative = true }
+OCIO_RS_LINK = "static"
+OCIO_RS_ENABLE_REAL = "1"

.gitignore

@@ -121,5 +121,4 @@ tarpaulin-out/
 .env
 # CD packaging artifacts
 /*.dmg
-.cargo/config.toml
 perf.data