chore: bump rust and cargo to 1.68

[oddgrd]

Description of change

This bumps our pinned rust version for deployer to 1.68, as well as bumping our cargo deps to v1.68. There were some changes to how cargo handles test errors in 1.66, I need to double check to make sure I have implemented these changes correctly on our end. For reference: rust-lang/cargo#11028

How Has This Been Tested (if applicable)?

Ran deploy tests locally and made images.

TODO: test on unstable.

[oddgrd]

Old cargo shuttle logs <id> output when pre-deploy tests fail:

2023-03-21T14:02:50.913757144Z  INFO    Compiling hello-world v0.1.0 (/opt/shuttle/shuttle-builds/oddgrd-test-tests)
2023-03-21T14:02:51.213412297Z  INFO     Finished release [optimized] target(s) in 19.00s
2023-03-21T14:02:51.220835046Z  INFO      Running unittests src/main.rs (opt/shuttle/shuttle-builds/oddgrd-test-tests/target/release/deps/hello_world-1ea9078ef2886609)

2023-03-21T14:02:51.235743683Z  INFO Entering crashed state
2023-03-21T14:02:51.235794420Z ERROR {error="Pre-deployment test failure: Tests failed: process didn't exit successfully: `/opt/shuttle/shuttle-builds/oddgrd-test-tests/target/release/deps/hello_world-1ea9078ef2886609` (exit status: 101)"} shuttle_deployer::deployment::queue: service build encountered an error

New cargo shuttle logs <id> output:

2023-03-21T13:30:38.263625727Z  INFO    Compiling hello-world v0.1.0 (/opt/shuttle/shuttle-builds/hello-world-axum-app)
2023-03-21T13:30:38.521043845Z  INFO     Finished release [optimized] target(s) in 1m 05s
2023-03-21T13:30:38.529522303Z  INFO      Running unittests src/main.rs (opt/shuttle/shuttle-builds/hello-world-axum-app/target/release/deps/hello_world-6fec48c4baa26a5b)
2023-03-21T13:30:38.535992943Z  INFO error: test failed, to rerun pass `-p hello-world --bin hello-world`

2023-03-21T13:30:38.545670405Z  INFO Entering crashed state
2023-03-21T13:30:38.545773794Z ERROR {error="Pre-deployment test failure: The deployment's tests failed."} shuttle_deployer::deployment::queue: service build encountered an error

[chesedo]

LGTM

[jonaro00]

@oddgrd By the way, after this is releases, wont the recompilation issue just switch over so that it happens for users on Rust <=1.67 (or whichever version it was)?
On that note, would calling the user's installed cargo instead of bundling cargo be a goal worth pursuing? Or is the usage of cargo too integrated?