test: name the whole credential variable, not its prefix

Three cases asserted $FLAGSMITH_API_KEY where the CLI prints the
host-scoped name in full. The prefix matches either spelling, so they would
not have noticed the scoping being dropped — which is the thing that keeps a
credential from following a redirected apiUrl.

beep boop

Author: khvn26

internal/cmd/testdata/script/credentials.txtar

@@ -25,7 +25,7 @@ stderr 'FLAGSMITH_ACCESS_TOKEN'
 env $API_KEY_VAR=$MASTER_KEY
 env $TOKEN_VAR=$BEARER_TOKEN
 exec flagsmith auth status
-stdout '\$FLAGSMITH_API_KEY'
+stdout '\$'$API_KEY_VAR
 
 # Given: a server-side key offered in the variable meant for Master API keys
 # When: the credential is described
@@ -40,7 +40,7 @@ stderr 'FLAGSMITH_ENVIRONMENT_KEY'
 # Then: the environment wins, and says so
 env $API_KEY_VAR=$MASTER_KEY
 exec flagsmith auth status
-stdout '\$FLAGSMITH_API_KEY'
+stdout '\$'$API_KEY_VAR
 
 # Given: an expired OAuth credential in the keychain, and no environment credential
 # When: a command refreshes it
@@ -57,7 +57,7 @@ env $API_KEY_VAR=$MASTER_KEY
 exec flagsmith auth status
 stdout 'Master API key'
 stdout 'Acme'
-stdout '\$FLAGSMITH_API_KEY'
+stdout '\$'$API_KEY_VAR
 exec flagsmith auth token
 stdout '^'$MASTER_KEY'$'