github: also run tests on our MSRV (1.58)

This patch sets up an additional CI strategy running tests on 1.58, to
make sure we don't accidentally use newer features. I've only made it
run on one platform (Linux) to avoid wasting resources. There's of
course a small risk that we start depending on platform-specific APIs
from newer versions.

.github/workflows/build.yml

@@ -10,17 +10,31 @@ permissions: read-all
 
 jobs:
   build:
-    runs-on: ${{ matrix.operating-system }}
     strategy:
+      fail-fast: false
       matrix:
-        operating-system: [ ubuntu-latest, windows-latest, macos-latest ]
+        build: [linux, linux-msrv, macos, win]
+        include:
+        - build: linux
+          os: ubuntu-latest
+          rust_version: stable
+        - build: linux-msrv
+          os: ubuntu-latest
+          rust_version: 1.58
+        - build: macos
+          os: macos-latest
+          rust_version: stable
+        - build: win
+          os: windows-latest
+          rust_version: stable
+    runs-on: ${{ matrix.os }}
 
     steps:
     - uses: actions/checkout@dcd71f646680f2efd8db4afa5ad64fdcba30e748
-    - name: Install Rust (stable)
+    - name: Install Rust (${{ matrix.rust_version }})
       uses: actions-rs/toolchain@16499b5e05bf2e26879000db0c1d13f7e13fa3af
       with:
-        toolchain: stable
+        toolchain:  ${{ matrix.rust_version }}
         override: true
         profile: minimal
     - name: Build

Cargo.toml

@@ -3,7 +3,7 @@ name = "jujutsu"
 version = "0.4.0"
 authors = ["Martin von Zweigbergk <martinvonz@google.com>"]
 edition = "2021"
-rust-version = "1.58"
+rust-version = "1.58"  # Remember to update CI
 license = "Apache-2.0"
 description = "Jujutsu (an experimental VCS)"
 homepage = "https://github.com/martinvonz/jj"