ci: bump to go 1.20 (#375)

* Update CI Go version to 1.20/1.19
* Update `golangci-lint` to v1.51.2
* Update coverage collection to use new Go 1.20 tools
* Update coverage job to only run on latest Go version

Fixes #367

script/coverage

@@ -1,7 +1,17 @@
-#!/bin/bash -ex
+#!/bin/bash
+
+set -euo pipefail
 
 ext=".coverprofile"
 
+# Temporary working directory.
+workdir=$(mktemp -d)
+
+function cleanup() {
+    rm -rf "${workdir}"
+}
+trap cleanup EXIT
+
 # Clean existing coverage files.
 find . -name '*'${ext} | xargs rm -f
 
@@ -9,16 +19,27 @@ find . -name '*'${ext} | xargs rm -f
 go test -covermode=count -coverprofile=unittests${ext} ./...
 
 # Integration test coverage.
-coverprofiles=()
+#
+# Collect for each individual avo assembly generator.
+coverbase="${workdir}/cover"
+coversingle="${coverbase}/test"
+mkdir -p "${coversingle}"
+
+coverdirs=()
 for main in $(find . -name 'asm.go'); do
 	dir=$(dirname ${main})
 	name=$(basename ${dir})
-	coverprofile="${dir}/${name}${ext}"
-	./script/covermain ${main} ${coverprofile} > /dev/null
-	coverprofiles+=(${coverprofile})
+
+	coverdir="${coversingle}/${name}"
+	mkdir "${coverdir}"
+	GOCOVERDIR="${coverdir}" go run -tags=integration -cover "${main}" -out /dev/null
+
+	coverdirs+=("${coverdir}")
 done
 
-covertool merge --output integration${ext} ${coverprofiles[@]}
-
-# Merge.
-covertool merge --output all${ext} unittests${ext} integration${ext}
+# Merge integration test coverage.
+covermergeinputs=$(IFS=","; echo "${coverdirs[*]}")
+covermergeoutput="${coverbase}/merge"
+mkdir "${covermergeoutput}"
+go tool covdata merge -i="${covermergeinputs}" -o="${covermergeoutput}"
+go tool covdata textfmt -i="${covermergeoutput}" -o="integration${ext}"