pin proc-macro2 for msrv

Author: ModProg

.github/workflows/test.yml

@@ -65,6 +65,7 @@ jobs:
           cargo update -p zeroize --precise 1.6.0
           cargo update -p pretty_assertions --precise 1.4.0
           cargo update -p glob --precise 0.3.2
+          cargo update -p proc-macro2 --precise 1.0.66
       - name: Build
         run:
           cargo build --workspace ${{ matrix.features }}
@@ -126,6 +127,7 @@ jobs:
         run: |
           cargo update -p zeroize --precise 1.6.0
           cargo update -p pretty_assertions --precise 1.4.0
+          cargo update -p proc-macro2 --precise 1.0.66
       - name: Build
         run:
           cargo build --target thumbv6m-none-eabi ${{ matrix.features }} -p ensure-no-std