switch to a newer riscv toolchain image with a Dockerfile I can edit if I ever want

.github/workflows/build.yml

@@ -5,15 +5,13 @@ jobs:
     name: Build C code
     runs-on: ubuntu-latest
     container:
-      image: plincar/riscv-gnu-toolchain:2023_03_12
+      image: runtimeverificationinc/riscv-gnu-toolchain:ubuntu-jammy-2024.04.12
       # volumes:
       #   - my_docker_volume:/volume_mount
     steps:
       - uses: actions/checkout@v4
       - name: Check for executables
         run: |
-          # this really should have been added to path at install time when the container was made
-          export PATH=$PATH:/opt/riscv/gnu-toolchain/rv64/bin
           riscv64-unknown-elf-gcc --version
       - name: Build
         run: |
@@ -21,5 +19,4 @@ jobs:
           # ls -lah
           # cat Makefile
           # make test.hex
-          export PATH=$PATH:/opt/riscv/gnu-toolchain/rv64/bin
           riscv64-unknown-elf-as -march=rv32i -mabi=ilp32 test.S -o test.o