When using a +build paramete+r in the Properties Content field of the Inject environmental variables build step the plugin adds a '=' sign at the end.

Example:

1. Define a Text Parameter called: ENV_VARS_TEST
2. Add a build step "Inject environmental variables"
3. Set Properties Content to ${ENV_VARS_TEST}
4. Start a Build and set the ENV_VARS_TEST Build Parameter to $FOO_VAR=BAR

The Build Output:
[EnvInject] - Loading node environment variables.
[EnvInject] - Injecting environment variables from a build step.
{{[EnvInject] - Injecting as environment variables the properties content }}
$FOO_VAR=BAR=

See the "=" added at the end of the line.

Same happens if I define "FOO_VAR=BAR" (without the $ sign) in the build parameter. Then I get

[EnvInject] - Injecting as environment variables the properties content
FOO_VAR=BAR=

When I add the string "$FOO_VAR=BAR" directly into the "Inject environmental variables" build step "Properties Content" section I get the expected "$FOO_VAR=BAR", like so:
[EnvInject] - Injecting environment variables from a build step.
[EnvInject] - Injecting as environment variables the properties content
$FOO_VAR=BAR

As soon as I put a build parameter into the Properties Content section I get this behavior with the added "=" at the end.

I was also able to reproduce this in a fresh free-style created project with just this and a shell-echo buildstep

.