Fixed indentation in env.txt header

[AndrewVallette]

Description

The header for the env.txt file that is generated in write_env_txt() contains leading whitespace that libmamba is not able to parse. This causes it to use a blank string for a matchspec, resulting in the following critical error:

critical libmamba Invalid spec, no package name found:

This issue occurs because lstrip() is called on a multi-line f-string before dedent(), which causes leading whitespace to only be removed up to the beginning of the first non-blank line. Calling dedent() before lstrip() will ensure all leading whitespace is stripped out for all lines.

Without this fix, the header in write_env_txt() is written out as:

# This file may be used to create an environment using:
        # $ conda create --name <env> --file <this file>
        # platform: osx-arm64
        @EXPLICIT

After the fix, the header is written out without leading whitespace:

# This file may be used to create an environment using:
# $ conda create --name <env> --file <this file>
# platform: osx-arm64
@EXPLICIT

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[dbast]

@conda-bot check

[dbast]

Tested this and it indeed fixes the bug as described... Merging without news entry as this bug was not present in the last release.

[jaimergp]

Thanks!

I usually add the PR number to the news item that introduced the feature being fixed. In this case, the file is https://github.com/conda/constructor/blob/main/news/559-explicit-install, and I'd add it to the item in Enhancements.

Could you please address this in a new PR? Thanks!