ci: pin to memchr 2.6.2 for MSRV CI job

I botched the memchr 2.6 MSRV because it actually requires Rust 1.61 and
not Rust 1.60. This crate's MSRV is Rust 1.60, so pin memchr to a
version that works on Rust 1.60 (for x86-64 at least).

.github/workflows/ci.yml

@@ -142,6 +142,25 @@ jobs:
       uses: dtolnay/rust-toolchain@master
       with:
         toolchain: 1.60.0
+    # The memchr 2.6 release purportedly bumped its MSRV to Rust 1.60, but it
+    # turned out that on aarch64, it was using something that wasn't stabilized
+    # until Rust 1.61[1]. (This was an oversight on my part. I had previously
+    # thought everything I needed was on Rust 1.60.) To resolve that, I just
+    # bumped memchr's MSRV to 1.61. Since it was so soon after the memchr 2.6
+    # release, I treated this as a bugfix.
+    #
+    # But the regex crate's MSRV is at Rust 1.60, and it now depends on at
+    # least memchr 2.6 (to make use of its `alloc` feature). So we can't set
+    # a lower minimal version. And I can't just bump the MSRV in a patch
+    # release as a bug fix because regex 1.9 was released quite some time ago.
+    # I could just release regex 1.10 and bump the MSRV there, but eh, I don't
+    # want to put out another minor version release just for this.
+    #
+    # So... pin memchr to 2.6.2, which at least works on x86-64 on Rust 1.60.
+    #
+    # [1]: https://github.com/BurntSushi/memchr/issues/136
+    - name: Pin memchr to 2.6.2
+      run: cargo update -p memchr --precise 2.6.2
     - name: Basic build
       run: cargo build --verbose
     - name: Build docs