Recognize install snippets that follow environment variable declarations

[rnjudge]

Describe the Feature
It is common in Dockerfiles that build go apps to set some environment variables before running the actual snippet that might install go modules. For example:

RUN CGO_ENABLED=0 GOOS=linux go build -v -a -installsuffix cgo -o swag cmd/swag/main.go

For the above line, Tern currently does not recognize "go build" as a snippet to install modules, despite that snippet being listed in snippets.py. This issue proposes changes to the way Tern parses

Use Cases
See golang example above.

Implementation Changes
Changes to filter_install_commands() or other associated functions, like split_command(), to better filter through potential environment variables that might come before a snippet install command.

[ForgetMe17]

This is kind of bug for now, because our parser will consider
RUN CGO_ENABLED=0 GOOS=linux go build -v -a -installsuffix cgo -o swag cmd/swag/main.go
as a variable assignment. Here are the result.

{'content': 'CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -o app .',
 'variable': {'name': 'CGO_ENABLED',
              'value': '0 GOOS=linux go build -a -installsuffix cgo -o app .'}}

[ForgetMe17]

I can take on this issue! @rnjudge
We can modify parse_shell_variables_and_command() in tern\utils\general.py. We can loop on a search for the variable_pattern until we can not find the macthed case.
For command CGO_ENABLED=0 GOOS=linux go build -v -a -installsuffix cgo -o swag cmd/swag/main.go:

1. Extract CGO_ENABLED=0 by ^([A-Za-z_][A-Za-z0-9_]*)=(.*) and the remaining part should be GOOS=linux go build -v -a -installsuffix cgo -o swag cmd/swag/main.go.
2. Extract GOOS=linux by ^([A-Za-z_][A-Za-z0-9_]*)=(.*) and the remaining part should be go build -v -a -installsuffix cgo -o swag cmd/swag/main.go.
3. The remaining part does not contains a variable_pattern so it should be command.

Note that this method only works on the condition that the variable assignment are followed by command which should be a common case. The reason is that we are looking for a variable assignment from the start of command.

[nishakm]

@ForgetMe17 do you still want to work on this?

[ForgetMe17]

@nishakm I will try on it!