Add call to go mod tidy

Signed-off-by: Terry Howe <tlhowe@amazon.com>

Makefile

@@ -17,7 +17,7 @@ CLI_PKG     = $(PROJECT_PKG)/cmd/oras
 GIT_COMMIT  = $(shell git rev-parse HEAD)
 GIT_TAG     = $(shell git describe --tags --abbrev=0 --exact-match 2>/dev/null)
 GIT_DIRTY   = $(shell test -n "`git status --porcelain`" && echo "dirty" || echo "clean")
-GO=go1.20
+GO=go
 
 TARGET_OBJS ?= checksums.txt darwin_amd64.tar.gz darwin_arm64.tar.gz linux_amd64.tar.gz linux_arm64.tar.gz linux_armv7.tar.gz linux_s390x.tar.gz windows_amd64.zip
 
@@ -32,7 +32,7 @@ LDFLAGS += -X $(PROJECT_PKG)/internal/version.GitCommit=${GIT_COMMIT}
 LDFLAGS += -X $(PROJECT_PKG)/internal/version.GitTreeState=${GIT_DIRTY}
 
 .PHONY: test
-test: vendor check-encoding
+test: tidy vendor check-encoding
 	$(GO) test -race -v -coverprofile=coverage.txt -covermode=atomic ./...
 
 .PHONY: covhtml
@@ -103,6 +103,10 @@ check-encoding:
 fix-encoding:
 	find cmd internal -type f -name "*.go" -exec sed -i -e "s/\r//g" {} +
 
+.PHONY: tidy
+tidy:
+	GO111MODULE=on $(GO) mod tidy
+
 .PHONY: vendor
 vendor:
 	GO111MODULE=on $(GO) mod vendor