[skip changelog] Refactor and add TEST_LDFLAGS to test-unit-race and test-legacy tasks

[rsora]

Please check if the PR fulfills these requirements

• The PR has no duplicates (please search among the Pull Requests
  before creating one)
• The PR follows
  our contributing guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• UPGRADING.md has been updated with a migration guide (for breaking changes)

• What kind of change does this PR introduce?

The PR adds the TEST_LDFLAGS to test-unit-race and test-legacy tasks

• What is the current behavior?

The task test-unit-race was failing in checking if the correct version parameter are passed correctly during build time here https://github.com/arduino/arduino-cli/blob/master/version/version_test.go#L29

=== RUN   TestBuildInjectedInfo
    version_test.go:38: 
                Error Trace:    version_test.go:38
                Error:          Not equal: 
                                expected: "0.0.0-test.preview"
                                actual  : "0.0.0-git"
                            
                                Diff:
                                --- Expected
                                +++ Actual
                                @@ -1 +1 @@
                                -0.0.0-test.preview
                                +0.0.0-git
                Test:           TestBuildInjectedInfo
--- FAIL: TestBuildInjectedInfo (0.00s)
FAIL
coverage: 1.0% of statements in ./...
FAIL    github.com/arduino/arduino-cli/version  0.226s
FAIL

• What is the new behavior?

Passing now to the task commands the TEST_LDFLAGS, the test runs successfully

• Does this PR introduce a breaking change, and is
  titled accordingly?

• Other information:

Bonus: Bumped the go version in the go.mod to 1.16 here c3358a0

---

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[rsora]

@per1234 do you think that this is worth to port somehow in our asset repo?

[per1234]

It must be proof that nobody ever ran the test-unit-race task. Should we use the -race flag when the CI runs the unit tests? I guess it will make them take a little longer, but it doesn't seem to be significant, especially since the unit tests will always finish way before the integration test workflow run.

[rsora]

Should we use the -race flag when the CI runs the unit tests?

Sounds reasonable to me, I'll open another PR after discussing it with @cmaglie and @silvanocerza 👍