[lit-starter-ts] run test:prod errors with: Invalid parameter - =prod

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npm test:dev works, but npm test:prod gives the following error:
```
> @lit/lit-starter-ts@1.0.0 test:prod
> MODE=prod wtr
Invalid parameter - =prod
```
On Windows 10.
Not sure if this is related, but when I type MODE by itself a program runs:
```
C:\repos\lit-element-starter-ts-main1>MODE

Status for device COM3:
-----------------------
Baud: 1200
Parity: None
Data Bits: 7
Stop Bits: 1
Timeout: OFF
XON/XOFF: OFF
CTS handshaking: OFF
DSR handshaking: OFF
DSR sensitivity: OFF
DTR circuit: ON
RTS circuit: ON

Status for device CON:
----------------------
Lines: 9001
Columns: 120
Keyboard rate: 31
Keyboard delay: 1
Code page: 65001

C:\repos\lit-element-starter-ts-main1>MODE /?
Configures system devices.

Serial port: MODE COMm[:] [BAUD=b] [PARITY=p] [DATA=d] [STOP=s]
[to=on|off] [xon=on|off] [odsr=on|off]
[octs=on|off] [dtr=on|off|hs]
[rts=on|off|hs|tg] [idsr=on|off]

Device Status: MODE [device] [/STATUS]

Redirect printing: MODE LPTn[:]=COMm[:]

Select code page: MODE CON[:] CP SELECT=yyy

Code page status: MODE CON[:] CP [/STATUS]

Display mode: MODE CON[:] [COLS=c] [LINES=n]

Typematic rate: MODE CON[:] [RATE=r DELAY=d]
```

Inspect the npm scripts invoked by test:dev and test:prod, then reproduce npm test:prod on Windows 10 to compare how MODE=prod is interpreted. Done means the production test command runs successfully on Windows without the Invalid parameter - =prod error.

Written by the indexing model from the issue text.