dotnet run -lp Profile uses env variables from the specified profile and ignores the values set in the current environment.
This is true even when the environment variable is explicitly set via an environment directive, e.g. dotnet [env:X=Y] -lp Profile.
As a result, there is no way to override an environment variable value set in the profile.

dotnet-watch launches projects via dotnet run. When invoked by Aspire DCP server it needs to implement Aspire launch profile semantics, which require any explicitly specified environment variable value to override the value in the launch profile. To implement this protocol correctly we need to be able to invoke dotnet run with specified env variable values overridden.

Possible options:

• change run to only set env vars from the LP if there isn't a value already set in the executing environment [breaking change]
• change run to only set env vars from the LP if there isn't a value already set in the by [env:X=Y] directive [less breaking]
• change run to have a new -e/--environment KEY=VALUE option that can be used to override the value [almost not breaking]

Note:
Command line arguments passed explicitly on dotnet run command line override those specified in launch profile. This is the desired behavior and it seems environment variables should behave the same way.

Related env variable directive issue:
#40484