Downgrade `rlimit` to 0.7.

Upstream 0.8.2 has broken compilation with Rust 1.53; versions
0.8.{0,1} have been yanked.

Possibly by the time the next arti version comes out, they'll have
fixed this situation, or we'll have upgraded our MSRV.

Upstream issue at https://github.com/Nugine/rlimit/issues/42 .

Cargo.lock

@@ -2566,9 +2566,9 @@ dependencies = [
 
 [[package]]
 name = "rlimit"
-version = "0.8.1"
+version = "0.7.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "0fbcc6f9672090a4060a6de8d0fb2d4acd381363cda5b7049a26e2b32464f8ad"
+checksum = "347703a5ae47adf1e693144157be231dde38c72bd485925cae7407ad3e52480b"
 dependencies = [
  "libc",
 ]

crates/arti-testing/Cargo.toml

@@ -39,7 +39,7 @@ tracing = "0.1.18"
 notify = "4.0"
 pin-project = "1"
 rand = "0.8"
-rlimit = "0.8.0"
+rlimit = "0.7"
 serde = { version = "1.0.103", features = ["derive"] }
 tracing-subscriber = { version = "0.3.0", features = ["env-filter"] }
 tokio = { version = "1.7", features = ["signal", "macros"] }

crates/arti/Cargo.toml

@@ -38,7 +38,7 @@ futures = "0.3.14"
 tracing = "0.1.18"
 notify = "4.0"
 once_cell = { version = "1", optional = true }
-rlimit = "0.8.0"
+rlimit = "0.7"
 serde = { version = "1.0.103", features = ["derive"] }
 tracing-subscriber = { version = "0.3.0", features = ["env-filter"] }
 tokio-crate = { package = "tokio", version = "1.7", optional = true, features = ["signal"] }