Improper multi-line templating of pre-build configuration

Checklist

I've looked through the issues and pull requests for similar reports

Describe your issue

Using a multi-line string in Cross.toml's pre-build field causes $CROSS_DEB_ARCH to not be properly replaced with the current architecture.

What target(s) are you cross-compiling for?

armv7-unknown-linux-gnueabihf

Which operating system is the host (e.g computer cross is on) running?

macOS
Windows
Linux / BSD
other OS (specify in description)

What architecture is the host?

x86_64 / AMD64
arm32
arm64 (including Mac M1)

What container engine is cross using?

docker
podman
other container engine (specify in description)

cross version

cross 0.2.5

Example

Here are two failing configurations:

[build]
pre-build = [
    "dpkg --add-architecture $CROSS_DEB_ARCH",
    "apt-get update",
    """apt-get --assume-yes --no-install-recommends install
    libssl-dev:$CROSS_DEB_ARCH
    libcairo2-dev:$CROSS_DEB_ARCH
    """,
]

[build]
pre-build = [
    "dpkg --add-architecture $CROSS_DEB_ARCH",
    "apt-get update",
    """apt-get --assume-yes --no-install-recommends install \
    libssl-dev:$CROSS_DEB_ARCH \
    libcairo2-dev:$CROSS_DEB_ARCH \
    """,
]

Here is an equivalent, working configuration using a simple string:

[build]
pre-build = [
    "dpkg --add-architecture $CROSS_DEB_ARCH",
    "apt-get update",
    "apt-get --assume-yes --no-install-recommends install libssl-dev:$CROSS_DEB_ARCH libcairo2-dev:$CROSS_DEB_ARCH",
]

Expected behavior:

# Functional
apt-get --assume-yes --no-install-recommends install
    libssl-dev:armhf
    libcairo2-dev:armhf

Current behavior:

# Errors: Unknown architecture
apt-get --assume-yes --no-install-recommends install
    libssl-dev:$CROSS_DEB_ARCH
    libcairo2-dev:$CROSS_DEB_ARCH

Additional information / notes

No response

Interesting, this is how it's put into the Dockerfile

cross/src/docker/shared.rs

Lines 169 to 172 in d8631fe

    
                           FROM {image} 
        
                           ARG CROSS_DEB_ARCH= 
        
                           ARG CROSS_CMD 
        
                           RUN eval "${{CROSS_CMD}}""#

and I think the problem is a combination of how it's expanded in quotemarks and

cross/src/docker/shared.rs

Line 180 in d8631fe

Some(("CROSS_CMD", pre_build.join("\n"))),

will have to investigate

	FROM {image}
	ARG CROSS_DEB_ARCH=
	ARG CROSS_CMD
	RUN eval "${{CROSS_CMD}}""#