fix coverage generation

[TroyKomodo]

Stop rustc from emitted --codegen=instrument-coverage when the target does not have coverage enabled via --instrumentation_filter

[TroyKomodo]

I am not actually sure why the unit tests dont pass,

From looking at it

rustc \
	'test/unit/pipelined_compilation/bin.rs '--crate-name=bin' '--crate-type=bin' '--error-format=human' \
	'--out-dir=bazel-out/k8-fastbuild-ST-1c8a4433bbf5/bin/test/unit/pipelined_compilation' \
	'--codegen=opt-level=0' '--codegen=debuginfo=0' '--codegen=strip=none' '--remap-path-prefix=${pwd}=' \
	'--emit=link=bazel-out/k8-fastbuild-ST-1c8a4433bbf5/bin/test/unit/pipelined_compilation/bin' '--emit=dep-info'  \
	'--color=always' '--target=x86_64-unknown-linux-gnu'  \
	'-L rust_toolchain/lib/rustlib/x86_64-unknown-linux-gnu/lib' '--edition=2021' '-Cembed-bitcode=no' \
	'--codegen=linker=/usr/bin/gcc' '--codegen=link-arg=--coverage' '--codegen=link-arg=-fuse-ld=gold' \
	'--codegen=link-arg=-B/usr/bin' '--codegen=link-arg=-Wl,-no-as-needed' '--codegen=link-arg=-Wl,-z,relro,-z,now'  \
	'--codegen=link-arg=-pass-exit-codes' '--codegen=link-arg=-Wl,--push-state,-as-needed' '--codegen=link-arg=-lstdc++' \
	'--codegen=link-arg=-Wl,--pop-state' '--codegen=link-arg=-Wl,--push-state,-as-needed' '--codegen=link-arg=-lm' \
	'--codegen=link-arg=-Wl,--pop-state' \
	'--extern=second=./bin/test/unit/pipelined_compilation/libsecond-416833743.rlib' \
	'-Ldependency=bazel-out/k8-fastbuild-ST-1c8a4433bbf5/bin/test/unit/pipelined_compilation' \
	'-Ldependency=bazel-out/k8-opt-exec-ST-742c84c296cd/bin/test/unit/pipelined_compilation' \
	'--codegen=instrument-coverage' \
	'--sysroot=./rust_toolchain' \

Use --sandbox_debug to see verbose messages from the sandbox and retain the sandbox build root for debugging
error[E0463]: can't find crate for `second`
 --> test/unit/pipelined_compilation/bin.rs:1:5
  |
1 | use second::fun;
  |     ^^^^^^ can't find crate

error: aborting due to 1 previous error

Which doesn't make sense to me because we are clearly passing --extern=second=...

I also dont think my changes effect the dependency resolution graph so again I'm not sure why rustc is behaving this way, perhaps its showing an error for not finding the crate but the actual issue is it not finding the llvm library the crate needs for coverage?