Print the command that is run if we are --verbose

[Enselic]

This makes it easier to debug problems when you are doing tricky things like constructing the example command at #360.

Here is what the new --verbose output says if run with that example:

Running `cd "." && CARGO_TARGET_DIR="target-bisector-nightly-2023-03-31-x86_64-unknown-linux-gnu" RUSTUP_TOOLCHAIN="bisector-nightly-2023-03-31-x86_64-unknown-linux-gnu" "bash" "-c" "rustc -Cpasses=lint -Cinstrument-coverage --crate-type=dylib tests/ui/issues/issue-85461.rs 2>&1 | grep Undefined"`

Here is what it looks like with context:

checking the start range to find a passing nightly
installing nightly-2023-03-31
testing...
Running `cd "." && CARGO_TARGET_DIR="target-bisector-nightly-2023-03-31-x86_64-unknown-linux-gnu" RUSTUP_TOOLCHAIN="bisector-nightly-2023-03-31-x86_64-unknown-linux-gnu" "bash" "-c" "rustc -Cpasses=lint -Cinstrument-coverage --crate-type=dylib tests/ui/issues/issue-85461.rs 2>&1 | grep Undefined"`
RESULT: nightly-2023-03-31, ===> Script returned success

checking the end range to verify it does not pass
installing nightly-2023-12-21
testing...
Running `cd "." && CARGO_TARGET_DIR="target-bisector-nightly-2023-12-21-x86_64-unknown-linux-gnu" RUSTUP_TOOLCHAIN="bisector-nightly-2023-12-21-x86_64-unknown-linux-gnu" "bash" "-c" "rustc -Cpasses=lint -Cinstrument-coverage --crate-type=dylib tests/ui/issues/issue-85461.rs 2>&1 | grep Undefined"`
RESULT: nightly-2023-12-21, ===> Script returned error

133 versions remaining to test after this (roughly 8 steps)
installing nightly-2023-08-10
testing...
Running `cd "." && CARGO_TARGET_DIR="target-bisector-nightly-2023-08-10-x86_64-unknown-linux-gnu" RUSTUP_TOOLCHAIN="bisector-nightly-2023-08-10-x86_64-unknown-linux-gnu" "bash" "-c" "rustc -Cpasses=lint -Cinstrument-coverage --crate-type=dylib tests/ui/issues/issue-85461.rs 2>&1 | grep Undefined"`
RESULT: nightly-2023-08-10, ===> Script returned error

[ehuss]

Thanks!