cras: Declare "language: rust"

[afq984]

The coverage builder complains that rust stdlib sources is missing.

Step #5: �[0;31merror: /workspace/out/libfuzzer-coverage-x86_64/rustc/187b8131d4f760f856b214fce34534903276f2ef/library/core/src/panic.rs: No such file or directory
Step #5: �[0m�[0;31mwarning: The file '/rustc/187b8131d4f760f856b214fce34534903276f2ef/library/core/src/panic.rs' isn't covered.
Step #5: �[0m[2023-10-04 06:50:42,764 DEBUG] Finished generating per-file code coverage summary.
Step #5: [2023-10-04 06:50:42,764 DEBUG] Generating file view html index file as: "/workspace/out/libfuzzer-coverage-x86_64/report/linux/file_view_index.html".
Step #5: Traceback (most recent call last):
Step #5:   File "/opt/code_coverage/coverage_utils.py", line 829, in <module>
Step #5:     sys.exit(Main())
Step #5:   File "/opt/code_coverage/coverage_utils.py", line 823, in Main
Step #5:     return _CmdPostProcess(args)
Step #5:   File "/opt/code_coverage/coverage_utils.py", line 780, in _CmdPostProcess
Step #5:     processor.PrepareHtmlReport()
Step #5:   File "/opt/code_coverage/coverage_utils.py", line 577, in PrepareHtmlReport
Step #5:     self.GenerateFileViewHtmlIndexFile(per_file_coverage_summary,
Step #5:   File "/opt/code_coverage/coverage_utils.py", line 450, in GenerateFileViewHtmlIndexFile
Step #5:     self.GetCoverageHtmlReportPathForFile(file_path),
Step #5:   File "/opt/code_coverage/coverage_utils.py", line 422, in GetCoverageHtmlReportPathForFile
Step #5:     assert os.path.isfile(
Step #5: AssertionError: "/rustc/187b8131d4f760f856b214fce34534903276f2ef/library/core/src/panic.rs" is not a file.

I think the sources would be copied in base-builder if we declare cras as a rust project:

oss-fuzz/infra/base-images/base-builder/compile

Lines 226 to 231 in d514fac

	if [ "$FUZZING_LANGUAGE" = "rust" ]; then
	# Copy rust std lib to its path with a hash.
	export rustch=`rustc --version --verbose | grep commit-hash | cut -d' ' -f2`
	mkdir -p /rustc/$rustch/
	cp -r /rust/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/ /rustc/$rustch/
	fi

Fixes: https://crbug.com/oss-fuzz/62974

[github-actions]

afq984 is either the primary contact or is in the CCs list of projects/cras.
afq984 has previously contributed to projects/cras. The previous PR was #10786