Avoid parking_lot bustage with recent rustc nightlies.

[jdm]

• ./mach build -d does not report any errors
• ./mach test-tidy does not report any errors
• These changes fix #26644
• There are tests for these changes

[highfive]

Heads up! This PR modifies the following files:

• @asajeffrey: components/script/Cargo.toml
• @KiChjang: components/script/Cargo.toml
• @emilio: components/style/Cargo.toml, components/layout/Cargo.toml

[highfive]

Warning

• These commits modify style, layout, and script code, but no tests are modified. Please consider adding a test!

[asajeffrey]

@bors-servo r+

[bors-servo]

📌 Commit bd6c9d9 has been approved by asajeffrey

[CYBAI]

We're able to remove metal from the duplicated list 🎉

./Cargo.lock:1: duplicates for `metal` are allowed, but only single version found
	The following packages depend on version 0.18.0 from 'crates.io':

Also, looks like this nightly doesn't have rustfmt 👀 (I checked rustup-component-history; rustfmt is missing for few days...)

error: �(Bcomponent 'rustfmt' for target 'x86_64-unknown-linux-gnu' is unavailable for download for channel nightly-2020-05-20
Sometimes not all components are available in any given nightly.
error: �(Bbacktrace:
error: �(Bstack backtrace:
   0: error_chain::backtrace::imp::InternalBacktrace::new
   1: rustup::dist::manifestation::Manifestation::update
   2: rustup::toolchain::Toolchain::add_component
   3: rustup_init::rustup_mode::main
   4: rustup_init::run_rustup_inner
   5: rustup_init::main
   6: std::rt::lang_start::{{closure}}
   7: main
   8: __libc_start_main
   9: <unknown>
error: �(B'cargo-fmt' is not installed for the toolchain 'nightly-2020-05-20-x86_64-unknown-linux-gnu'
To install, run `rustup component add rustfmt`
error: �(Bbacktrace:
error: �(Bstack backtrace:
   0: error_chain::backtrace::imp::InternalBacktrace::new
   1: rustup::toolchain::Toolchain::create_command
   2: rustup_init::run_rustup_inner
   3: rustup_init::main
   4: std::rt::lang_start::{{closure}}
   5: main
   6: __libc_start_main
   7: <unknown>
clang-format not installed. Skipping CPP formatting.
Run `./mach fmt` to fix the formatting

[jdm]

@bors-servo r-

[jdm]

@bors-servo r=asajeffrey

[bors-servo]

📌 Commit 1da40b5 has been approved by asajeffrey

[bors-servo]

⌛ Testing commit 1da40b5 with merge f331373...

[bors-servo]

☀️ Test successful - status-taskcluster
Approved by: asajeffrey
Pushing f331373 to master...