chore: migrate to Rust 2024

Removes the use of `set_var`, since it isn't thread safe.

Cargo.toml

@@ -3,7 +3,7 @@ name = "cosmic-session"
 description = "The session manager for the COSMIC desktop environment"
 version = "0.1.0"
 license = "GPL-3.0-only"
-edition = "2021"
+edition = "2024"
 authors = ["Lucy <lucy@system76.com>"]
 publish = false
 
@@ -25,20 +25,20 @@ sendfd = { version = "0.4", features = ["tokio"] }
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 tokio = { version = "1", features = [
-  "fs",
-  "io-util",
-  "io-std",
-  "macros",
-  "net",
-  "parking_lot",
-  "process",
-  "rt",
-  "signal",
-  "sync",
-  "time",
+    "fs",
+    "io-util",
+    "io-std",
+    "macros",
+    "net",
+    "parking_lot",
+    "process",
+    "rt",
+    "signal",
+    "sync",
+    "time",
 ] }
 zbus_systemd = { version = "0.25701.0", optional = true, features = [
-  "systemd1",
+    "systemd1",
 ] }
 tokio-util = "0.7"
 tracing = "0.1"