POSIX-like shells such as bash offer a way to set environment variables in a command-scoped way, simply by prepending <varName>=<value> pairs directly to a command, as the following example demonstrates:
$ foo=bar bash -c 'echo "[$foo]"'; echo "[$foo]"
[bar]
[]
foo=bar defines environment variable foo for the bash -c '...' child process only; the next command - echo ... - does not see this variable.
PowerShell has NO equivalent construct.
The best you can do is to define the environment variable of interest first, in a separate statement, using ;, PowerShell's statement separator. Any external utility you invoke thereafter - which invariably runs in a child process - will see it, but note that the environment variable will remain in effect in the current PowerShell session, unless you manually remove it:
# Set the env. variable, call the command that should see it,
# remove it afterwards.
PS> $env:foo = 'bar'; bash -c 'echo "[$foo]"'; $env:foo = $null
[bar]
Note how $env:foo = $null i.e., setting the environment variable to $null is the same as removing it; alternatively, you could all Remove-Item env:foo
If you also want to restore a pre-existing value afterwards:
$env:foo = 'original'
# Temporarily change $env:foo to a different value, invoke the
# program that should see it, then restore the previous value.
& { $org, $env:foo = $env:foo, 'bar'; bash -c 'echo "[$foo]"'; $env:foo = $org }
$env:foo
The above yields:
[bar]
original
showing that while the bash process saw the temporary value, bar, the original value of $env:foo was restored afterwards.
Also note another important difference:
In POSIX-like shells, environment variables are implicitly surfaced as shell variables - they share the one and only namespace the shell has for variables.
By contrast, PowerShell surfaces environment variables only via the $env:<varName> namespace (e.g., $env:foo), which is distinct from the (prefix-less) namespace for PowerShell's own variables (e.g., $foo).
