Add the TARGET_LIBS environment variable for rustc CI testing

[msizanoen1]

Needed to fix the test failure in rust-lang/rust#66158.

See rust-lang/rust#66158 (comment)

r? @Manishearth

changelog: none

[tesuji]

Ping @flip1995
@msizanoen1 you might need to rebase against upstream master.

[tesuji]

build failed because of format check. Could you run ./util/dev fmt?

[Manishearth]

Do you think this will work when cross compiling? I'm strongly inclined to just disable these tests when running rustc ci.

[mati865]

Tools tests aren't ran when cross compiling right now.
In fact they are ran only on x86_64-gnu-tools and x86_64-msvc-tools which are native but user could enable them for local build.

[Manishearth]

Oh, right!

Anyway, I found a better fix for this in #4784, we really shouldn't be linking to clippy_lints in those tests anyway.

[msizanoen1]

Actually even with that PR you will still get the regex and serde crate not found error.

[msizanoen1]

About the tools are tested only when building natively assumption, the rustbuild build system always pass the --target flags so the target rlibs are put in a separate directory.

[msizanoen1]

Cross testing is not a goal of this PR.

[Manishearth]

the must_use isn't necessary

[Manishearth]

@Brors r+

will land for now and maybe make minor fixes later

[msizanoen1]

@Brors r+

Who's @Brors? You probably mean @bors

[Manishearth]

@bors r+

[bors]

📌 Commit 7d2e813 has been approved by Manishearth

[bors]

⌛ Testing commit 7d2e813 with merge af3090c...

[bors]

💔 Test failed - checks-travis

[flip1995]

@bors retry

Changelog was missing

[bors]

⌛ Testing commit 7d2e813 with merge 692b260...

[bors]

☀️ Test successful - checks-travis, status-appveyor
Approved by: Manishearth
Pushing 692b260 to master...