[rust/coverage] Use new flag for coverage instrumentation

> warning: `-Z instrument-coverage` is deprecated; use `-C
instrument-coverage`

Test: above warning is absent when running:
    $ m CLANG_COVERAGE=true NATIVE_COVERAGE_PATHS=* rust
Change-Id: I8687c04534db3c8a67f209b93ed13d54c2f86dc8
diff --git a/rust/coverage.go b/rust/coverage.go
index 651ce6e..5ea481f 100644
--- a/rust/coverage.go
+++ b/rust/coverage.go
@@ -56,7 +56,7 @@
 		flags.Coverage = true
 		coverage := ctx.GetDirectDepWithTag(CovLibraryName, cc.CoverageDepTag).(cc.LinkableInterface)
 		flags.RustFlags = append(flags.RustFlags,
-			"-Z instrument-coverage", "-g")
+			"-C instrument-coverage", "-g")
 		flags.LinkFlags = append(flags.LinkFlags,
 			profileInstrFlag, "-g", coverage.OutputFile().Path().String(), "-Wl,--wrap,open")
 		deps.StaticLibs = append(deps.StaticLibs, coverage.OutputFile().Path())