Update workflows to Ubuntu 20.04, as 18.04 does not have the RISC-V toolchain we need.

.github/workflows/ci.yml

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 6631920..f62bca8 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -16,9 +16,9 @@
     # Using ubuntu-latest can cause breakage when ubuntu-latest is updated to
     # point at a new Ubuntu version. Instead, explicitly specify the version, so
     # we can update when we need to. This *could* break if we don't update it
-    # until support for 18.04 is dropped, but it is likely we'll have a reason
+    # until support for 20.04 is dropped, but it is likely we'll have a reason
     # to update to a newer Ubuntu before then anyway.
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
 
     steps:
       # Clones a single commit from the libtock-rs repository. The commit cloned

.github/workflows/size-diff.yml

diff --git a/.github/workflows/size-diff.yml b/.github/workflows/size-diff.yml
index b991c5f..33a0870 100644
--- a/.github/workflows/size-diff.yml
+++ b/.github/workflows/size-diff.yml
@@ -17,9 +17,9 @@
     # Using ubuntu-latest can cause breakage when ubuntu-latest is updated to
     # point at a new Ubuntu version. Instead, explicitly specify the version, so
     # we can update when we need to. This *could* break if we don't update it
-    # until support for 18.04 is dropped, but it is likely we'll have a reason
+    # until support for 20.04 is dropped, but it is likely we'll have a reason
     # to update to a newer Ubuntu before then anyway.
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
 
     steps:
       # Clones a single commit from the libtock-rs repository. The commit cloned