Remove toolchain setup steps from our GitHub Actions workflows.

Rustup will not automatically install the correct targets, so we no longer need the `rustup target add` steps. After this, the `actions-rs/toolchain` step doesn't give us much, and is currently broken (https://github.com/actions-rs/toolchain/issues/126), so I removed that step as well.

.github/workflows/artifacts.yml

diff --git a/.github/workflows/artifacts.yml b/.github/workflows/artifacts.yml
index 527e418..94a8b10 100644
--- a/.github/workflows/artifacts.yml
+++ b/.github/workflows/artifacts.yml
@@ -10,16 +10,8 @@
       - name: Checkout repository
         uses: actions/checkout@v2
 
-      - name: Install Rust toolchain
-        uses: actions-rs/toolchain@v1.0.6
-        with:
-          profile: minimal
-
       - name: Install dependencies
         run: |
-          rustup target add thumbv7em-none-eabi
-          rustup target add riscv32imac-unknown-none-elf
-          rustup target add riscv32imc-unknown-none-elf
           cargo install elf2tab --version 0.4.0
 
       - name: Build Hello World

.github/workflows/ci.yml

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index ad0738e..5bdae5f 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -31,15 +31,6 @@
         with:
           submodules: true
 
-      # Install a Rust toolchain with the components necessary to build
-      # libtock-rs and its examples. This action doesn't seem to be able to
-      # install toolchains for multiple targets, so I add the targets later in
-      # the "Build and Test" step.
-      - name: Install Rust toolchain
-        uses: actions-rs/toolchain@v1.0.6
-        with:
-          profile: minimal
-
       # The main test step. We let the makefile do most of the work because the
       # makefile can be tested locally. We experimentally determined that -j2 is
       # optimal for the Azure Standard_DS2_v2 VM, which is the VM type used by
@@ -47,6 +38,5 @@
       - name: Build and Test
         run: |
           cd "${GITHUB_WORKSPACE}"
-          rustup target add riscv32imc-unknown-none-elf thumbv7em-none-eabi
           make -j2 setup
           make -j2 test

.github/workflows/size-diff.yml

diff --git a/.github/workflows/size-diff.yml b/.github/workflows/size-diff.yml
index 2c3b597..7a8e010 100644
--- a/.github/workflows/size-diff.yml
+++ b/.github/workflows/size-diff.yml
@@ -29,15 +29,6 @@
       - name: Clone repository
         uses: actions/checkout@v2.3.0
 
-      # Install a Rust toolchain with the components necessary to run
-      # `print-sizes`. This action doesn't seem to be able to install toolchains
-      # for multiple targets, so I add the targets later in the "size report"
-      # step.
-      - name: Install Rust toolchain
-        uses: actions-rs/toolchain@v1.0.6
-        with:
-          profile: minimal
-
       # The main diff script. Stores the sizes of the example binaries for both
       # the merge commit and the target branch. We display the diff in a
       # separate step to make it easy to navigate to in the GitHub Actions UI.
@@ -46,13 +37,11 @@
           UPSTREAM_REMOTE_NAME="${UPSTREAM_REMOTE_NAME:-origin}"
           GITHUB_BASE_REF="${GITHUB_BASE_REF:-master}"
           cd "${GITHUB_WORKSPACE}"
-          rustup target add riscv32imc-unknown-none-elf thumbv7em-none-eabi
           make -j2 examples  # The VM this runs on has 2 logical cores.
           cargo run --release -p print_sizes >'${{runner.temp}}/merge-sizes'
           git remote set-branches "${UPSTREAM_REMOTE_NAME}" "${GITHUB_BASE_REF}"
           git fetch --depth=1 "${UPSTREAM_REMOTE_NAME}" "${GITHUB_BASE_REF}"
           git checkout "${UPSTREAM_REMOTE_NAME}/${GITHUB_BASE_REF}"
-          rustup target add riscv32imc-unknown-none-elf thumbv7em-none-eabi
           make -j2 examples
           cargo run --release -p print_sizes >'${{runner.temp}}/base-sizes'