Add missing config.toml

Signed-off-by: Ming-Wei Shih <mishih@microsoft.com>

.cargo/config.toml

@@ -0,0 +1,11 @@
+[build]
+target = "x86_64-unknown-linux-musl"
+
+# Flags to enable code-coverage for all builds.
+# These can be removed later.
+rustflags = ["-Cinstrument-coverage"]
+incremental = true
+
+[env]
+# Name of coverage instrumentation log file.
+LLVM_PROFILE_FILE="target/cargo-test-%p-%m.profraw"