[cli] fix `new` command tests, pass in optional target dir (#197)

* [cli] use temp dir for `cargo contract new` tests

* [cli] pass in temp dir to build command

* [cli] rustfmt

* [cli] remove some spaces

* [cli] rustfmt

* [cli] fix unused warnings

* [cli] TEMPORARY: see if test works on CI without nightly flag

* [cli] build: check if nightly toolchain is the default

* [cli] TEMPORARY: dump cargo output of build command test

* [cli] TEMPORARY: fix error in las commit

* [cli] TEMPORARY: add some diagnostics to see if .cargo/config is there

* [cli] TEMPORARY: use cat

* [cli] fix merge compilation error

* [cli] change ls and cat commands to current_dir

* [cli] fix troubleshooting logging

* [cli] TEMPORARY: comment out RUSTFLAGS for troubleshooting

* [cli] add link-dead-code args to top level .cargo/config

Avoids RUSTFLAGS env var overriding contract project's rust flags

* [cli] revert temporary CI build troubleshooting

* [cli] make rustflags not target specific

* [cli] only link-dead-code in CI

* [cli] TEMP: output .cargo/config

* [cli] printf instead of echo

* [ci] create cargo config in $CARGO_HOME instead of workspace root

* [ci] fix cargo_home path

* [ci] overwrite rather than append cargo home config

* [ci] restore original RUSTFLAGS, ignore failing build test

* [ci] remove stray echo