Unquoted bool's makes dotenv output format barf

Given:

$ sops -d --pgp <key> test.yml

foo: false
bar: true

However, if we try to dump the same file as a dotenv, we get the following:

$ sops -d --pgp <key> --output-type dotenv test.yml
panic: interface conversion: interface {} is bool, not string

goroutine 1 [running]:
go.mozilla.org/sops/v3/stores/dotenv.(*Store).EmitPlainFile(0x23483b0, 0xc0001214b8, 0x1, 0x1, 0x0, 0x1da61b0, 0xc0005d4120, 0xc0005dd200, 0x20)
	/private/tmp/sops-20210408-88082-6m5hl1/sops-3.7.1/stores/dotenv/store.go:122 +0x40f
main.decrypt(0x1da61b0, 0xc0005d4120, 0x9b1bc98, 0x23483b0, 0x1db2998, 0x23483b0, 0xc000174e40, 0x20, 0x0, 0x0, ...)
	/private/tmp/sops-20210408-88082-6m5hl1/sops-3.7.1/cmd/sops/decrypt.go:47 +0x278
main.main.func8(0xc000286c60, 0x0, 0x0)
	/private/tmp/sops-20210408-88082-6m5hl1/sops-3.7.1/cmd/sops/main.go:809 +0x2d85
gopkg.in/urfave/cli%2ev1.HandleAction(0x1a6afc0, 0x1c28bb8, 0xc000286c60, 0x0, 0x0)
	/Users/brew/Library/Caches/Homebrew/go_mod_cache/pkg/mod/gopkg.in/urfave/cli.v1@v1.20.0/app.go:490 +0x82
gopkg.in/urfave/cli%2ev1.(*App).Run(0xc0005009c0, 0xc00012c000, 0x7, 0x7, 0x0, 0x0)
	/Users/brew/Library/Caches/Homebrew/go_mod_cache/pkg/mod/gopkg.in/urfave/cli.v1@v1.20.0/app.go:264 +0x5f5
main.main()
	/private/tmp/sops-20210408-88082-6m5hl1/sops-3.7.1/cmd/sops/main.go:987 +0x3885

Quoting the boolean fixes the issue for dotenv output:

$ sops -d --pgp <key> test.yml
foo: "true"
bar: "true"

$ sops -d --pgp <key> --output-type dotenv test.yml
foo=true
bar=true

Start in stores/dotenv/store.go at Store.EmitPlainFile, the entry point named in the panic trace, and reproduce the failure with YAML boolean values such as foo: false and bar: true. Verify that decrypting with --output-type dotenv no longer panics and emits valid dotenv key-value lines.