chore: reduce GHA dependabot frequency

Author: robjtede

.cspell.yml

@@ -1,14 +1,16 @@
 version: "0.2"
 words:
   - btreemap
+  - clippy
   - deque
   - deserializable
   - docsrs
   - dotenv
   - indoc
   - ints
   - maplit
-  - msrv
+  - MSRV
+  - nextest
   - nixpkgs
   - noncomposite
   - nums
@@ -19,11 +21,11 @@ words:
   - println
   - refpath
   - reqwest
+  - RUSTDOCFLAGS
   - rustfmt
   - rustup
   - semver
   - serde
-  - spdx
   - strs
   - struct
   - structs

.github/dependabot.yml

@@ -3,7 +3,7 @@ updates:
   - package-ecosystem: github-actions
     directory: /
     schedule:
-      interval: weekly
+      interval: monthly
   - package-ecosystem: cargo
     directory: /
     schedule:

rustfmt.toml renamed to .rustfmt.toml

@@ -1,6 +1,8 @@
 _list:
     @just --list
 
+toolchain := ""
+
 msrv := ```
     cargo metadata --format-version=1 \
     | jq -r 'first(.packages[] | select(.source == null and .rust_version)) | .rust_version' \
@@ -41,12 +43,15 @@ clippy:
 # Downgrade dev-dependencies necessary to run MSRV checks/tests.
 [private]
 downgrade-for-msrv:
+    cargo {{ toolchain }} update -p=idna_adapter --precise=1.2.0 # next ver: 1.82.0
     cargo update -p=litemap --precise=0.7.4 # next ver: 1.81.0
     cargo update -p=zerofrom --precise=0.1.5 # next ver: 1.81.0
 
 # Test workspace using MSRV.
 [group("test")]
-test-msrv: downgrade-for-msrv (test msrv_rustup)
+test-msrv:
+    @just toolchain={{ msrv_rustup }} downgrade-for-msrv
+    @just toolchain={{ msrv_rustup }} test
 
 # Test workspace.
 [group("test")]