Use Rust 0.73.0 (#118)

<!-- The PR description should answer 2 (maybe 3) important questions:
-->

### What

Bump nix flake, bump Rust toolchain, fix suggestions.

### How

<!-- How is it trying to accomplish it (what are the implementation
steps)? -->

crates/tests/databases-tests/src/capabilities_tests.rs

@@ -1,7 +1,4 @@
-#[cfg(test)]
-mod capabilities_tests {
-    #[tokio::test]
-    async fn get_capabilities() {
-        insta::assert_json_snapshot!(ndc_postgres::capabilities::get_capabilities());
-    }
+#[tokio::test]
+async fn get_capabilities() {
+    insta::assert_json_snapshot!(ndc_postgres::capabilities::get_capabilities());
 }

crates/tests/tests-common/src/schemas.rs

@@ -1,3 +1,4 @@
+use std::fmt::Write;
 /// Checks that a given value conforms to the schema generated by `schemars`.
 ///
 /// Panics with a human-readable error if the value does not conform, or if the
@@ -17,14 +18,14 @@ pub fn check_value_conforms_to_schema<T: schemars::JsonSchema>(value: serde_json
         Err(errors) => {
             panic!(
                 "The configuration does not conform to the schema.\n{}",
-                errors
-                    .map(|error| {
-                        format!(
-                            "{}\ninstance path: {}\nschema path:   {}\n\n",
-                            error, error.instance_path, error.schema_path
-                        )
-                    })
-                    .collect::<String>()
+                errors.fold(String::new(), |mut str, error| {
+                    let _ = write!(
+                        str,
+                        "{}\ninstance path: {}\nschema path:   {}\n\n",
+                        error, error.instance_path, error.schema_path
+                    );
+                    str
+                })
             )
         }
     }

rust-toolchain.toml

@@ -1,4 +1,4 @@
 [toolchain]
-channel = "1.72.1"
+channel = "1.73.0"
 profile = "default"  # see https://rust-lang.github.io/rustup/concepts/profiles.html
 components = ["rust-analyzer", "rust-src"]  # see https://rust-lang.github.io/rustup/concepts/components.html