fix lttf dashboard action - install pnpm #149

Job	Run time
Floating Dependencies	35s
Tests	37s
ember-lts-3.16	15m 1s
ember-lts-3.20	47s
ember-lts-3.24	1m 0s
embroider-safe	44s
ember-lts-4.8	44s
ember-release	48s
ember-beta	1m 6s
ember-canary	1m 14s
ember-lts-4.12	39s
ember-classic	53s
ember-lts-4.4	46s
ember-lts-3.28	48s
ember-release-no-deprecations	1m 6s
no-deprecations	1m 2s
embroider-optimized	1m 16s
	29m 6s

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